Re: Trouble with using group commitlog_sync

[Bowen Song via user]

Okay, that proves I was wrong on the client side bottleneck.

On 24/04/2024 17:55, Nathan Marz wrote:
I tried running two client processes in parallel and the numbers were 
unchanged. The max throughput is still a single client doing 10 
in-flight BatchStatement containing 100 inserts.


On Tue, Apr 23, 2024 at 10:24 PM Bowen Song via user 
 wrote:


You might have run into the bottleneck of the driver's IO thread.
Try increase the driver's connections-per-server limit to 2 or 3
if you've only got 1 server in the cluster. Or alternatively, run
two client processes in parallel.


On 24/04/2024 07:19, Nathan Marz wrote:

Tried it again with one more client thread, and that had no
effect on performance. This is unsurprising as there's only 2 CPU
on this node and they were already at 100%. These were good
ideas, but I'm still unable to even match the performance of
batch commit mode with group commit mode.

On Tue, Apr 23, 2024 at 12:46 PM Bowen Song via user
 wrote:

To achieve 10k loop iterations per second, each iteration
must take 0.1 milliseconds or less. Considering that each
iteration needs to lock and unlock the semaphore (two
syscalls) and make network requests (more syscalls), that's a
lots of context switches. It may a bit too much to ask for a
single thread. I would suggest try multi-threading or
multi-processing, and see if the combined insert rate is higher.

I should also note that executeAsync() also has implicit
limits on the number of in-flight requests, which default to
1024 requests per connection and 1 connection per server. See

https://docs.datastax.com/en/developer/java-driver/4.17/manual/core/pooling/


On 23/04/2024 23:18, Nathan Marz wrote:

It's using the async API, so why would it need multiple
threads? Using the exact same approach I'm able to get 38k /
second with periodic commitlog_sync. For what it's worth, I
do see 100% CPU utilization in every single one of these tests.

On Tue, Apr 23, 2024 at 11:01 AM Bowen Song via user
 wrote:

Have you checked the thread CPU utilisation of the
client side? You likely will need more than one thread
to do insertion in a loop to achieve tens of thousands
of inserts per second.


On 23/04/2024 21:55, Nathan Marz wrote:

Thanks for the explanation.

I tried again with commitlog_sync_group_window at 2ms,
concurrent_writes at 512, and doing 1000 individual
inserts at a time with the same loop + semaphore
approach. This only nets 9k / second.

I got much higher throughput for the other modes with
BatchStatement of 100 inserts rather than 100x more
individual inserts.

On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user
 wrote:

I suspect you are abusing batch statements. Batch
statements should only be used where atomicity or
isolation is needed. Using batch statements won't
make inserting multiple partitions faster. In fact,
it often will make that slower.

Also, the liner relationship between
commitlog_sync_group_window and write throughput is
expected. That's because the max number of
uncompleted writes is limited by the write
concurrency, and a write is not considered
"complete" before it is synced to disk when
commitlog sync is in group or batch mode. That
means within each interval, only limited number of
writes can be done. The ways to increase that
including: add more nodes, sync commitlog at
shorter intervals and allow more concurrent writes.


On 23/04/2024 20:43, Nathan Marz wrote:

Thanks. I raised concurrent_writes to 128 and
set commitlog_sync_group_window to 20ms. This
causes a single execute of a BatchStatement
containing 100 inserts to succeed. However, the
throughput I'm seeing is atrocious.

With these settings, I'm executing 10
BatchStatement concurrently at a time using the
semaphore + loop approach I showed in my first
message. So as requests complete, more are sent
out such that there are 10 in-flight at a time.
Each BatchStatement has 100 individual inserts.
I'm seeing only 730 inserts / second. Again, with
periodic mode I see 38k / second and with batch I
see 14k / second. My expectation was that group
commit mode throughput would be somewhere between

Re: Trouble with using group commitlog_sync

[Nathan Marz]

I tried running two client processes in parallel and the numbers were
unchanged. The max throughput is still a single client doing 10 in-flight
BatchStatement containing 100 inserts.

On Tue, Apr 23, 2024 at 10:24 PM Bowen Song via user <
user@cassandra.apache.org> wrote:

> You might have run into the bottleneck of the driver's IO thread. Try
> increase the driver's connections-per-server limit to 2 or 3 if you've only
> got 1 server in the cluster. Or alternatively, run two client processes in
> parallel.
>
>
> On 24/04/2024 07:19, Nathan Marz wrote:
>
> Tried it again with one more client thread, and that had no effect on
> performance. This is unsurprising as there's only 2 CPU on this node and
> they were already at 100%. These were good ideas, but I'm still unable to
> even match the performance of batch commit mode with group commit mode.
>
> On Tue, Apr 23, 2024 at 12:46 PM Bowen Song via user <
> user@cassandra.apache.org> wrote:
>
>> To achieve 10k loop iterations per second, each iteration must take 0.1
>> milliseconds or less. Considering that each iteration needs to lock and
>> unlock the semaphore (two syscalls) and make network requests (more
>> syscalls), that's a lots of context switches. It may a bit too much to ask
>> for a single thread. I would suggest try multi-threading or
>> multi-processing, and see if the combined insert rate is higher.
>>
>> I should also note that executeAsync() also has implicit limits on the
>> number of in-flight requests, which default to 1024 requests per connection
>> and 1 connection per server. See
>> https://docs.datastax.com/en/developer/java-driver/4.17/manual/core/pooling/
>>
>>
>> On 23/04/2024 23:18, Nathan Marz wrote:
>>
>> It's using the async API, so why would it need multiple threads? Using
>> the exact same approach I'm able to get 38k / second with periodic
>> commitlog_sync. For what it's worth, I do see 100% CPU utilization in every
>> single one of these tests.
>>
>> On Tue, Apr 23, 2024 at 11:01 AM Bowen Song via user <
>> user@cassandra.apache.org> wrote:
>>
>>> Have you checked the thread CPU utilisation of the client side? You
>>> likely will need more than one thread to do insertion in a loop to achieve
>>> tens of thousands of inserts per second.
>>>
>>>
>>> On 23/04/2024 21:55, Nathan Marz wrote:
>>>
>>> Thanks for the explanation.
>>>
>>> I tried again with commitlog_sync_group_window at 2ms, concurrent_writes
>>> at 512, and doing 1000 individual inserts at a time with the same loop +
>>> semaphore approach. This only nets 9k / second.
>>>
>>> I got much higher throughput for the other modes with BatchStatement of
>>> 100 inserts rather than 100x more individual inserts.
>>>
>>> On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user <
>>> user@cassandra.apache.org> wrote:
>>>
 I suspect you are abusing batch statements. Batch statements should
 only be used where atomicity or isolation is needed. Using batch statements
 won't make inserting multiple partitions faster. In fact, it often will
 make that slower.

 Also, the liner relationship between commitlog_sync_group_window and
 write throughput is expected. That's because the max number of uncompleted
 writes is limited by the write concurrency, and a write is not considered
 "complete" before it is synced to disk when commitlog sync is in group or
 batch mode. That means within each interval, only limited number of writes
 can be done. The ways to increase that including: add more nodes, sync
 commitlog at shorter intervals and allow more concurrent writes.


 On 23/04/2024 20:43, Nathan Marz wrote:

 Thanks. I raised concurrent_writes to 128 and
 set commitlog_sync_group_window to 20ms. This causes a single execute of a
 BatchStatement containing 100 inserts to succeed. However, the throughput
 I'm seeing is atrocious.

 With these settings, I'm executing 10 BatchStatement concurrently at a
 time using the semaphore + loop approach I showed in my first message. So
 as requests complete, more are sent out such that there are 10 in-flight at
 a time. Each BatchStatement has 100 individual inserts. I'm seeing only 730
 inserts / second. Again, with periodic mode I see 38k / second and with
 batch I see 14k / second. My expectation was that group commit mode
 throughput would be somewhere between those two.

 If I set commitlog_sync_group_window to 100ms, the throughput drops to
 14 / second.

 If I set commitlog_sync_group_window to 10ms, the throughput increases
 to 1587 / second.

 If I set commitlog_sync_group_window to 5ms, the throughput increases
 to 3200 / second.

 If I set commitlog_sync_group_window to 1ms, the throughput increases
 to 13k / second, which is slightly less than batch commit mode.

 Is group commit mode supposed to have better performance than batch
 mode?


 On Tue, Apr 23, 

Re: Trouble with using group commitlog_sync

[Bowen Song via user]

You might have run into the bottleneck of the driver's IO thread. Try 
increase the driver's connections-per-server limit to 2 or 3 if you've 
only got 1 server in the cluster. Or alternatively, run two client 
processes in parallel.



On 24/04/2024 07:19, Nathan Marz wrote:
Tried it again with one more client thread, and that had no effect on 
performance. This is unsurprising as there's only 2 CPU on this node 
and they were already at 100%. These were good ideas, but I'm still 
unable to even match the performance of batch commit mode with group 
commit mode.


On Tue, Apr 23, 2024 at 12:46 PM Bowen Song via user 
 wrote:


To achieve 10k loop iterations per second, each iteration must
take 0.1 milliseconds or less. Considering that each iteration
needs to lock and unlock the semaphore (two syscalls) and make
network requests (more syscalls), that's a lots of context
switches. It may a bit too much to ask for a single thread. I
would suggest try multi-threading or multi-processing, and see if
the combined insert rate is higher.

I should also note that executeAsync() also has implicit limits on
the number of in-flight requests, which default to 1024 requests
per connection and 1 connection per server. See
https://docs.datastax.com/en/developer/java-driver/4.17/manual/core/pooling/


On 23/04/2024 23:18, Nathan Marz wrote:

It's using the async API, so why would it need multiple threads?
Using the exact same approach I'm able to get 38k / second with
periodic commitlog_sync. For what it's worth, I do see 100% CPU
utilization in every single one of these tests.

On Tue, Apr 23, 2024 at 11:01 AM Bowen Song via user
 wrote:

Have you checked the thread CPU utilisation of the client
side? You likely will need more than one thread to do
insertion in a loop to achieve tens of thousands of inserts
per second.


On 23/04/2024 21:55, Nathan Marz wrote:

Thanks for the explanation.

I tried again with commitlog_sync_group_window at 2ms,
concurrent_writes at 512, and doing 1000 individual inserts
at a time with the same loop + semaphore approach. This only
nets 9k / second.

I got much higher throughput for the other modes with
BatchStatement of 100 inserts rather than 100x more
individual inserts.

On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user
 wrote:

I suspect you are abusing batch statements. Batch
statements should only be used where atomicity or
isolation is needed. Using batch statements won't make
inserting multiple partitions faster. In fact, it often
will make that slower.

Also, the liner relationship between
commitlog_sync_group_window and write throughput is
expected. That's because the max number of uncompleted
writes is limited by the write concurrency, and a write
is not considered "complete" before it is synced to disk
when commitlog sync is in group or batch mode. That
means within each interval, only limited number of
writes can be done. The ways to increase that including:
add more nodes, sync commitlog at shorter intervals and
allow more concurrent writes.


On 23/04/2024 20:43, Nathan Marz wrote:

Thanks. I raised concurrent_writes to 128 and
set commitlog_sync_group_window to 20ms. This causes a
single execute of a BatchStatement containing 100
inserts to succeed. However, the throughput I'm seeing
is atrocious.

With these settings, I'm executing 10 BatchStatement
concurrently at a time using the semaphore + loop
approach I showed in my first message. So as requests
complete, more are sent out such that there are 10
in-flight at a time. Each BatchStatement has 100
individual inserts. I'm seeing only 730 inserts /
second. Again, with periodic mode I see 38k / second
and with batch I see 14k / second. My expectation was
that group commit mode throughput would be somewhere
between those two.

If I set commitlog_sync_group_window to 100ms, the
throughput drops to 14 / second.

If I set commitlog_sync_group_window to 10ms, the
throughput increases to 1587 / second.

If I set commitlog_sync_group_window to 5ms, the
throughput increases to 3200 / second.

If I set commitlog_sync_group_window to 1ms, the
throughput increases to 13k / second, which is slightly
less than batch commit mode.

Is group commit mode supposed to have better
performance than batch mode?


On Tue, Apr 23, 2024 at 8:46 AM 

Re: Trouble with using group commitlog_sync

[Nathan Marz]

Tried it again with one more client thread, and that had no effect on
performance. This is unsurprising as there's only 2 CPU on this node and
they were already at 100%. These were good ideas, but I'm still unable to
even match the performance of batch commit mode with group commit mode.

On Tue, Apr 23, 2024 at 12:46 PM Bowen Song via user <
user@cassandra.apache.org> wrote:

> To achieve 10k loop iterations per second, each iteration must take 0.1
> milliseconds or less. Considering that each iteration needs to lock and
> unlock the semaphore (two syscalls) and make network requests (more
> syscalls), that's a lots of context switches. It may a bit too much to ask
> for a single thread. I would suggest try multi-threading or
> multi-processing, and see if the combined insert rate is higher.
>
> I should also note that executeAsync() also has implicit limits on the
> number of in-flight requests, which default to 1024 requests per connection
> and 1 connection per server. See
> https://docs.datastax.com/en/developer/java-driver/4.17/manual/core/pooling/
>
>
> On 23/04/2024 23:18, Nathan Marz wrote:
>
> It's using the async API, so why would it need multiple threads? Using the
> exact same approach I'm able to get 38k / second with periodic
> commitlog_sync. For what it's worth, I do see 100% CPU utilization in every
> single one of these tests.
>
> On Tue, Apr 23, 2024 at 11:01 AM Bowen Song via user <
> user@cassandra.apache.org> wrote:
>
>> Have you checked the thread CPU utilisation of the client side? You
>> likely will need more than one thread to do insertion in a loop to achieve
>> tens of thousands of inserts per second.
>>
>>
>> On 23/04/2024 21:55, Nathan Marz wrote:
>>
>> Thanks for the explanation.
>>
>> I tried again with commitlog_sync_group_window at 2ms, concurrent_writes
>> at 512, and doing 1000 individual inserts at a time with the same loop +
>> semaphore approach. This only nets 9k / second.
>>
>> I got much higher throughput for the other modes with BatchStatement of
>> 100 inserts rather than 100x more individual inserts.
>>
>> On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user <
>> user@cassandra.apache.org> wrote:
>>
>>> I suspect you are abusing batch statements. Batch statements should only
>>> be used where atomicity or isolation is needed. Using batch statements
>>> won't make inserting multiple partitions faster. In fact, it often will
>>> make that slower.
>>>
>>> Also, the liner relationship between commitlog_sync_group_window and
>>> write throughput is expected. That's because the max number of uncompleted
>>> writes is limited by the write concurrency, and a write is not considered
>>> "complete" before it is synced to disk when commitlog sync is in group or
>>> batch mode. That means within each interval, only limited number of writes
>>> can be done. The ways to increase that including: add more nodes, sync
>>> commitlog at shorter intervals and allow more concurrent writes.
>>>
>>>
>>> On 23/04/2024 20:43, Nathan Marz wrote:
>>>
>>> Thanks. I raised concurrent_writes to 128 and
>>> set commitlog_sync_group_window to 20ms. This causes a single execute of a
>>> BatchStatement containing 100 inserts to succeed. However, the throughput
>>> I'm seeing is atrocious.
>>>
>>> With these settings, I'm executing 10 BatchStatement concurrently at a
>>> time using the semaphore + loop approach I showed in my first message. So
>>> as requests complete, more are sent out such that there are 10 in-flight at
>>> a time. Each BatchStatement has 100 individual inserts. I'm seeing only 730
>>> inserts / second. Again, with periodic mode I see 38k / second and with
>>> batch I see 14k / second. My expectation was that group commit mode
>>> throughput would be somewhere between those two.
>>>
>>> If I set commitlog_sync_group_window to 100ms, the throughput drops to
>>> 14 / second.
>>>
>>> If I set commitlog_sync_group_window to 10ms, the throughput increases
>>> to 1587 / second.
>>>
>>> If I set commitlog_sync_group_window to 5ms, the throughput increases to
>>> 3200 / second.
>>>
>>> If I set commitlog_sync_group_window to 1ms, the throughput increases to
>>> 13k / second, which is slightly less than batch commit mode.
>>>
>>> Is group commit mode supposed to have better performance than batch mode?
>>>
>>>
>>> On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user <
>>> user@cassandra.apache.org> wrote:
>>>
 The default commitlog_sync_group_window is very long for SSDs. Try
 reduce it if you are using SSD-backed storage for the commit log. 10-15 ms
 is a good starting point. You may also want to increase the value of
 concurrent_writes, consider at least double or quadruple it from the
 default. You'll need even higher write concurrency for longer
 commitlog_sync_group_window.

 On 23/04/2024 19:26, Nathan Marz wrote:

 "batch" mode works fine. I'm having trouble with "group" mode. The only
 config for that is "commitlog_sync_group_window", 

Re: Trouble with using group commitlog_sync

[Bowen Song via user]

To achieve 10k loop iterations per second, each iteration must take 0.1 
milliseconds or less. Considering that each iteration needs to lock and 
unlock the semaphore (two syscalls) and make network requests (more 
syscalls), that's a lots of context switches. It may a bit too much to 
ask for a single thread. I would suggest try multi-threading or 
multi-processing, and see if the combined insert rate is higher.


I should also note that executeAsync() also has implicit limits on the 
number of in-flight requests, which default to 1024 requests per 
connection and 1 connection per server. See 
https://docs.datastax.com/en/developer/java-driver/4.17/manual/core/pooling/



On 23/04/2024 23:18, Nathan Marz wrote:
It's using the async API, so why would it need multiple threads? Using 
the exact same approach I'm able to get 38k / second with periodic 
commitlog_sync. For what it's worth, I do see 100% CPU utilization in 
every single one of these tests.


On Tue, Apr 23, 2024 at 11:01 AM Bowen Song via user 
 wrote:


Have you checked the thread CPU utilisation of the client side?
You likely will need more than one thread to do insertion in a
loop to achieve tens of thousands of inserts per second.


On 23/04/2024 21:55, Nathan Marz wrote:

Thanks for the explanation.

I tried again with commitlog_sync_group_window at 2ms,
concurrent_writes at 512, and doing 1000 individual inserts at a
time with the same loop + semaphore approach. This only nets 9k /
second.

I got much higher throughput for the other modes with
BatchStatement of 100 inserts rather than 100x more individual
inserts.

On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user
 wrote:

I suspect you are abusing batch statements. Batch statements
should only be used where atomicity or isolation is needed.
Using batch statements won't make inserting multiple
partitions faster. In fact, it often will make that slower.

Also, the liner relationship between
commitlog_sync_group_window and write throughput is expected.
That's because the max number of uncompleted writes is
limited by the write concurrency, and a write is not
considered "complete" before it is synced to disk when
commitlog sync is in group or batch mode. That means within
each interval, only limited number of writes can be done. The
ways to increase that including: add more nodes, sync
commitlog at shorter intervals and allow more concurrent writes.


On 23/04/2024 20:43, Nathan Marz wrote:

Thanks. I raised concurrent_writes to 128 and
set commitlog_sync_group_window to 20ms. This causes a
single execute of a BatchStatement containing 100 inserts to
succeed. However, the throughput I'm seeing is atrocious.

With these settings, I'm executing 10 BatchStatement
concurrently at a time using the semaphore + loop approach I
showed in my first message. So as requests complete, more
are sent out such that there are 10 in-flight at a time.
Each BatchStatement has 100 individual inserts. I'm seeing
only 730 inserts / second. Again, with periodic mode I see
38k / second and with batch I see 14k / second. My
expectation was that group commit mode throughput would be
somewhere between those two.

If I set commitlog_sync_group_window to 100ms, the
throughput drops to 14 / second.

If I set commitlog_sync_group_window to 10ms, the throughput
increases to 1587 / second.

If I set commitlog_sync_group_window to 5ms, the throughput
increases to 3200 / second.

If I set commitlog_sync_group_window to 1ms, the throughput
increases to 13k / second, which is slightly less than batch
commit mode.

Is group commit mode supposed to have better performance
than batch mode?


On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user
 wrote:

The default commitlog_sync_group_window is very long for
SSDs. Try reduce it if you are using SSD-backed storage
for the commit log. 10-15 ms is a good starting point.
You may also want to increase the value of
concurrent_writes, consider at least double or quadruple
it from the default. You'll need even higher write
concurrency for longer commitlog_sync_group_window.


On 23/04/2024 19:26, Nathan Marz wrote:

"batch" mode works fine. I'm having trouble with
"group" mode. The only config for that is
"commitlog_sync_group_window", and I have that set to
the default 1000ms.

On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user
 wrote:

Why would you want to set
commitlog_sync_batch_window to 1 second long when
 

Re: Trouble with using group commitlog_sync

[Nathan Marz]

It's using the async API, so why would it need multiple threads? Using the
exact same approach I'm able to get 38k / second with periodic
commitlog_sync. For what it's worth, I do see 100% CPU utilization in every
single one of these tests.

On Tue, Apr 23, 2024 at 11:01 AM Bowen Song via user <
user@cassandra.apache.org> wrote:

> Have you checked the thread CPU utilisation of the client side? You likely
> will need more than one thread to do insertion in a loop to achieve tens of
> thousands of inserts per second.
>
>
> On 23/04/2024 21:55, Nathan Marz wrote:
>
> Thanks for the explanation.
>
> I tried again with commitlog_sync_group_window at 2ms, concurrent_writes
> at 512, and doing 1000 individual inserts at a time with the same loop +
> semaphore approach. This only nets 9k / second.
>
> I got much higher throughput for the other modes with BatchStatement of
> 100 inserts rather than 100x more individual inserts.
>
> On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user <
> user@cassandra.apache.org> wrote:
>
>> I suspect you are abusing batch statements. Batch statements should only
>> be used where atomicity or isolation is needed. Using batch statements
>> won't make inserting multiple partitions faster. In fact, it often will
>> make that slower.
>>
>> Also, the liner relationship between commitlog_sync_group_window and
>> write throughput is expected. That's because the max number of uncompleted
>> writes is limited by the write concurrency, and a write is not considered
>> "complete" before it is synced to disk when commitlog sync is in group or
>> batch mode. That means within each interval, only limited number of writes
>> can be done. The ways to increase that including: add more nodes, sync
>> commitlog at shorter intervals and allow more concurrent writes.
>>
>>
>> On 23/04/2024 20:43, Nathan Marz wrote:
>>
>> Thanks. I raised concurrent_writes to 128 and
>> set commitlog_sync_group_window to 20ms. This causes a single execute of a
>> BatchStatement containing 100 inserts to succeed. However, the throughput
>> I'm seeing is atrocious.
>>
>> With these settings, I'm executing 10 BatchStatement concurrently at a
>> time using the semaphore + loop approach I showed in my first message. So
>> as requests complete, more are sent out such that there are 10 in-flight at
>> a time. Each BatchStatement has 100 individual inserts. I'm seeing only 730
>> inserts / second. Again, with periodic mode I see 38k / second and with
>> batch I see 14k / second. My expectation was that group commit mode
>> throughput would be somewhere between those two.
>>
>> If I set commitlog_sync_group_window to 100ms, the throughput drops to 14
>> / second.
>>
>> If I set commitlog_sync_group_window to 10ms, the throughput increases to
>> 1587 / second.
>>
>> If I set commitlog_sync_group_window to 5ms, the throughput increases to
>> 3200 / second.
>>
>> If I set commitlog_sync_group_window to 1ms, the throughput increases to
>> 13k / second, which is slightly less than batch commit mode.
>>
>> Is group commit mode supposed to have better performance than batch mode?
>>
>>
>> On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user <
>> user@cassandra.apache.org> wrote:
>>
>>> The default commitlog_sync_group_window is very long for SSDs. Try
>>> reduce it if you are using SSD-backed storage for the commit log. 10-15 ms
>>> is a good starting point. You may also want to increase the value of
>>> concurrent_writes, consider at least double or quadruple it from the
>>> default. You'll need even higher write concurrency for longer
>>> commitlog_sync_group_window.
>>>
>>> On 23/04/2024 19:26, Nathan Marz wrote:
>>>
>>> "batch" mode works fine. I'm having trouble with "group" mode. The only
>>> config for that is "commitlog_sync_group_window", and I have that set to
>>> the default 1000ms.
>>>
>>> On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user <
>>> user@cassandra.apache.org> wrote:
>>>
 Why would you want to set commitlog_sync_batch_window to 1 second long
 when commitlog_sync is set to batch mode? The documentation
 
 on this says:

 *This window should be kept short because the writer threads will be
 unable to do extra work while waiting. You may need to increase
 concurrent_writes for the same reason*

 If you want to use batch mode, at least ensure
 commitlog_sync_batch_window is reasonably short. The default is 2
 millisecond.


 On 23/04/2024 18:32, Nathan Marz wrote:

 I'm doing some benchmarking of Cassandra on a single m6gd.large
 instance. It works fine with periodic or batch commitlog_sync options, but
 I'm having tons of issues when I change it to "group". I have
 "commitlog_sync_group_window" set to 1000ms.

 My client is doing writes like this (pseudocode):

 Semaphore sem = new Semaphore(numTickets);
 while(true) {

 

Re: Trouble with using group commitlog_sync

[Bowen Song via user]

Have you checked the thread CPU utilisation of the client side? You 
likely will need more than one thread to do insertion in a loop to 
achieve tens of thousands of inserts per second.



On 23/04/2024 21:55, Nathan Marz wrote:

Thanks for the explanation.

I tried again with commitlog_sync_group_window at 2ms, 
concurrent_writes at 512, and doing 1000 individual inserts at a time 
with the same loop + semaphore approach. This only nets 9k / second.


I got much higher throughput for the other modes with BatchStatement 
of 100 inserts rather than 100x more individual inserts.


On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user 
 wrote:


I suspect you are abusing batch statements. Batch statements
should only be used where atomicity or isolation is needed. Using
batch statements won't make inserting multiple partitions faster.
In fact, it often will make that slower.

Also, the liner relationship between commitlog_sync_group_window
and write throughput is expected. That's because the max number of
uncompleted writes is limited by the write concurrency, and a
write is not considered "complete" before it is synced to disk
when commitlog sync is in group or batch mode. That means within
each interval, only limited number of writes can be done. The ways
to increase that including: add more nodes, sync commitlog at
shorter intervals and allow more concurrent writes.


On 23/04/2024 20:43, Nathan Marz wrote:

Thanks. I raised concurrent_writes to 128 and
set commitlog_sync_group_window to 20ms. This causes a single
execute of a BatchStatement containing 100 inserts to succeed.
However, the throughput I'm seeing is atrocious.

With these settings, I'm executing 10 BatchStatement concurrently
at a time using the semaphore + loop approach I showed in my
first message. So as requests complete, more are sent out such
that there are 10 in-flight at a time. Each BatchStatement has
100 individual inserts. I'm seeing only 730 inserts / second.
Again, with periodic mode I see 38k / second and with batch I see
14k / second. My expectation was that group commit mode
throughput would be somewhere between those two.

If I set commitlog_sync_group_window to 100ms, the throughput
drops to 14 / second.

If I set commitlog_sync_group_window to 10ms, the throughput
increases to 1587 / second.

If I set commitlog_sync_group_window to 5ms, the throughput
increases to 3200 / second.

If I set commitlog_sync_group_window to 1ms, the throughput
increases to 13k / second, which is slightly less than batch
commit mode.

Is group commit mode supposed to have better performance than
batch mode?


On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user
 wrote:

The default commitlog_sync_group_window is very long for
SSDs. Try reduce it if you are using SSD-backed storage for
the commit log. 10-15 ms is a good starting point. You may
also want to increase the value of concurrent_writes,
consider at least double or quadruple it from the default.
You'll need even higher write concurrency for longer
commitlog_sync_group_window.


On 23/04/2024 19:26, Nathan Marz wrote:

"batch" mode works fine. I'm having trouble with "group"
mode. The only config for that is
"commitlog_sync_group_window", and I have that set to the
default 1000ms.

On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user
 wrote:

Why would you want to set commitlog_sync_batch_window to
1 second long when commitlog_sync is set to batch mode?
The documentation


on this says:

/This window should be kept short because the writer
threads will be unable to do extra work while
waiting. You may need to increase concurrent_writes
for the same reason/

If you want to use batch mode, at least ensure
commitlog_sync_batch_window is reasonably short. The
default is 2 millisecond.


On 23/04/2024 18:32, Nathan Marz wrote:

I'm doing some benchmarking of Cassandra on a single
m6gd.large instance. It works fine with periodic or
batch commitlog_sync options, but I'm having tons of
issues when I change it to "group". I have
"commitlog_sync_group_window" set to 1000ms.

My client is doing writes like this (pseudocode):

Semaphore sem = new Semaphore(numTickets);
while(true) {

sem.acquire();
session.executeAsync(insert.bind(genUUIDStr(),
genUUIDStr(), genUUIDStr())
            .whenComplete((t, u) -> sem.release())

}

Re: Trouble with using group commitlog_sync

[Nathan Marz]

Thanks for the explanation.

I tried again with commitlog_sync_group_window at 2ms, concurrent_writes at
512, and doing 1000 individual inserts at a time with the same loop +
semaphore approach. This only nets 9k / second.

I got much higher throughput for the other modes with BatchStatement of 100
inserts rather than 100x more individual inserts.

On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user <
user@cassandra.apache.org> wrote:

> I suspect you are abusing batch statements. Batch statements should only
> be used where atomicity or isolation is needed. Using batch statements
> won't make inserting multiple partitions faster. In fact, it often will
> make that slower.
>
> Also, the liner relationship between commitlog_sync_group_window and
> write throughput is expected. That's because the max number of uncompleted
> writes is limited by the write concurrency, and a write is not considered
> "complete" before it is synced to disk when commitlog sync is in group or
> batch mode. That means within each interval, only limited number of writes
> can be done. The ways to increase that including: add more nodes, sync
> commitlog at shorter intervals and allow more concurrent writes.
>
>
> On 23/04/2024 20:43, Nathan Marz wrote:
>
> Thanks. I raised concurrent_writes to 128 and
> set commitlog_sync_group_window to 20ms. This causes a single execute of a
> BatchStatement containing 100 inserts to succeed. However, the throughput
> I'm seeing is atrocious.
>
> With these settings, I'm executing 10 BatchStatement concurrently at a
> time using the semaphore + loop approach I showed in my first message. So
> as requests complete, more are sent out such that there are 10 in-flight at
> a time. Each BatchStatement has 100 individual inserts. I'm seeing only 730
> inserts / second. Again, with periodic mode I see 38k / second and with
> batch I see 14k / second. My expectation was that group commit mode
> throughput would be somewhere between those two.
>
> If I set commitlog_sync_group_window to 100ms, the throughput drops to 14
> / second.
>
> If I set commitlog_sync_group_window to 10ms, the throughput increases to
> 1587 / second.
>
> If I set commitlog_sync_group_window to 5ms, the throughput increases to
> 3200 / second.
>
> If I set commitlog_sync_group_window to 1ms, the throughput increases to
> 13k / second, which is slightly less than batch commit mode.
>
> Is group commit mode supposed to have better performance than batch mode?
>
>
> On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user <
> user@cassandra.apache.org> wrote:
>
>> The default commitlog_sync_group_window is very long for SSDs. Try
>> reduce it if you are using SSD-backed storage for the commit log. 10-15 ms
>> is a good starting point. You may also want to increase the value of
>> concurrent_writes, consider at least double or quadruple it from the
>> default. You'll need even higher write concurrency for longer
>> commitlog_sync_group_window.
>>
>> On 23/04/2024 19:26, Nathan Marz wrote:
>>
>> "batch" mode works fine. I'm having trouble with "group" mode. The only
>> config for that is "commitlog_sync_group_window", and I have that set to
>> the default 1000ms.
>>
>> On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user <
>> user@cassandra.apache.org> wrote:
>>
>>> Why would you want to set commitlog_sync_batch_window to 1 second long
>>> when commitlog_sync is set to batch mode? The documentation
>>> 
>>> on this says:
>>>
>>> *This window should be kept short because the writer threads will be
>>> unable to do extra work while waiting. You may need to increase
>>> concurrent_writes for the same reason*
>>>
>>> If you want to use batch mode, at least ensure
>>> commitlog_sync_batch_window is reasonably short. The default is 2
>>> millisecond.
>>>
>>>
>>> On 23/04/2024 18:32, Nathan Marz wrote:
>>>
>>> I'm doing some benchmarking of Cassandra on a single m6gd.large
>>> instance. It works fine with periodic or batch commitlog_sync options, but
>>> I'm having tons of issues when I change it to "group". I have
>>> "commitlog_sync_group_window" set to 1000ms.
>>>
>>> My client is doing writes like this (pseudocode):
>>>
>>> Semaphore sem = new Semaphore(numTickets);
>>> while(true) {
>>>
>>> sem.acquire();
>>> session.executeAsync(insert.bind(genUUIDStr(), genUUIDStr(),
>>> genUUIDStr())
>>> .whenComplete((t, u) -> sem.release())
>>>
>>> }
>>>
>>> If I set numTickets higher than 20, I get tons of timeout errors.
>>>
>>> I've also tried doing single commands with BatchStatement with many
>>> inserts at a time, and that fails with timeout when the batch size gets
>>> more than 20.
>>>
>>> Increasing the write request timeout in cassandra.yaml makes it time out
>>> at slightly higher numbers of concurrent requests.
>>>
>>> With periodic I'm able to get about 38k writes / second, and with batch
>>> I'm able to get about 14k / second.
>>>
>>> Any tips on 

Re: Trouble with using group commitlog_sync

[Bowen Song via user]

I suspect you are abusing batch statements. Batch statements should only 
be used where atomicity or isolation is needed. Using batch statements 
won't make inserting multiple partitions faster. In fact, it often will 
make that slower.


Also, the liner relationship between commitlog_sync_group_window and 
write throughput is expected. That's because the max number of 
uncompleted writes is limited by the write concurrency, and a write is 
not considered "complete" before it is synced to disk when commitlog 
sync is in group or batch mode. That means within each interval, only 
limited number of writes can be done. The ways to increase that 
including: add more nodes, sync commitlog at shorter intervals and allow 
more concurrent writes.



On 23/04/2024 20:43, Nathan Marz wrote:
Thanks. I raised concurrent_writes to 128 and 
set commitlog_sync_group_window to 20ms. This causes a single execute 
of a BatchStatement containing 100 inserts to succeed. However, the 
throughput I'm seeing is atrocious.


With these settings, I'm executing 10 BatchStatement concurrently at a 
time using the semaphore + loop approach I showed in my first message. 
So as requests complete, more are sent out such that there are 10 
in-flight at a time. Each BatchStatement has 100 individual inserts. 
I'm seeing only 730 inserts / second. Again, with periodic mode I see 
38k / second and with batch I see 14k / second. My expectation was 
that group commit mode throughput would be somewhere between those two.


If I set commitlog_sync_group_window to 100ms, the throughput drops to 
14 / second.


If I set commitlog_sync_group_window to 10ms, the throughput increases 
to 1587 / second.


If I set commitlog_sync_group_window to 5ms, the throughput increases 
to 3200 / second.


If I set commitlog_sync_group_window to 1ms, the throughput increases 
to 13k / second, which is slightly less than batch commit mode.


Is group commit mode supposed to have better performance than batch mode?


On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user 
 wrote:


The default commitlog_sync_group_window is very long for SSDs. Try
reduce it if you are using SSD-backed storage for the commit log.
10-15 ms is a good starting point. You may also want to increase
the value of concurrent_writes, consider at least double or
quadruple it from the default. You'll need even higher write
concurrency for longer commitlog_sync_group_window.


On 23/04/2024 19:26, Nathan Marz wrote:

"batch" mode works fine. I'm having trouble with "group" mode.
The only config for that is "commitlog_sync_group_window", and I
have that set to the default 1000ms.

On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user
 wrote:

Why would you want to set commitlog_sync_batch_window to 1
second long when commitlog_sync is set to batch mode? The
documentation


on this says:

/This window should be kept short because the writer
threads will be unable to do extra work while waiting.
You may need to increase concurrent_writes for the same
reason/

If you want to use batch mode, at least ensure
commitlog_sync_batch_window is reasonably short. The default
is 2 millisecond.


On 23/04/2024 18:32, Nathan Marz wrote:

I'm doing some benchmarking of Cassandra on a single
m6gd.large instance. It works fine with periodic or batch
commitlog_sync options, but I'm having tons of issues when I
change it to "group". I have "commitlog_sync_group_window"
set to 1000ms.

My client is doing writes like this (pseudocode):

Semaphore sem = new Semaphore(numTickets);
while(true) {

sem.acquire();
session.executeAsync(insert.bind(genUUIDStr(),
genUUIDStr(), genUUIDStr())
            .whenComplete((t, u) -> sem.release())

}

If I set numTickets higher than 20, I get tons of timeout
errors.

I've also tried doing single commands with BatchStatement
with many inserts at a time, and that fails with timeout
when the batch size gets more than 20.

Increasing the write request timeout in cassandra.yaml makes
it time out at slightly higher numbers of concurrent requests.

With periodic I'm able to get about 38k writes / second, and
with batch I'm able to get about 14k / second.

Any tips on what I should be doing to get group
commitlog_sync to work properly? I didn't expect to have to
do anything other than change the config.

Re: Trouble with using group commitlog_sync

[Nathan Marz]

Thanks. I raised concurrent_writes to 128 and
set commitlog_sync_group_window to 20ms. This causes a single execute of a
BatchStatement containing 100 inserts to succeed. However, the throughput
I'm seeing is atrocious.

With these settings, I'm executing 10 BatchStatement concurrently at a time
using the semaphore + loop approach I showed in my first message. So as
requests complete, more are sent out such that there are 10 in-flight at a
time. Each BatchStatement has 100 individual inserts. I'm seeing only 730
inserts / second. Again, with periodic mode I see 38k / second and with
batch I see 14k / second. My expectation was that group commit mode
throughput would be somewhere between those two.

If I set commitlog_sync_group_window to 100ms, the throughput drops to 14 /
second.

If I set commitlog_sync_group_window to 10ms, the throughput increases to
1587 / second.

If I set commitlog_sync_group_window to 5ms, the throughput increases to
3200 / second.

If I set commitlog_sync_group_window to 1ms, the throughput increases to
13k / second, which is slightly less than batch commit mode.

Is group commit mode supposed to have better performance than batch mode?


On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user <
user@cassandra.apache.org> wrote:

> The default commitlog_sync_group_window is very long for SSDs. Try reduce
> it if you are using SSD-backed storage for the commit log. 10-15 ms is a
> good starting point. You may also want to increase the value of
> concurrent_writes, consider at least double or quadruple it from the
> default. You'll need even higher write concurrency for longer
> commitlog_sync_group_window.
>
> On 23/04/2024 19:26, Nathan Marz wrote:
>
> "batch" mode works fine. I'm having trouble with "group" mode. The only
> config for that is "commitlog_sync_group_window", and I have that set to
> the default 1000ms.
>
> On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user <
> user@cassandra.apache.org> wrote:
>
>> Why would you want to set commitlog_sync_batch_window to 1 second long
>> when commitlog_sync is set to batch mode? The documentation
>> 
>> on this says:
>>
>> *This window should be kept short because the writer threads will be
>> unable to do extra work while waiting. You may need to increase
>> concurrent_writes for the same reason*
>>
>> If you want to use batch mode, at least ensure
>> commitlog_sync_batch_window is reasonably short. The default is 2
>> millisecond.
>>
>>
>> On 23/04/2024 18:32, Nathan Marz wrote:
>>
>> I'm doing some benchmarking of Cassandra on a single m6gd.large instance.
>> It works fine with periodic or batch commitlog_sync options, but I'm having
>> tons of issues when I change it to "group". I have
>> "commitlog_sync_group_window" set to 1000ms.
>>
>> My client is doing writes like this (pseudocode):
>>
>> Semaphore sem = new Semaphore(numTickets);
>> while(true) {
>>
>> sem.acquire();
>> session.executeAsync(insert.bind(genUUIDStr(), genUUIDStr(), genUUIDStr())
>> .whenComplete((t, u) -> sem.release())
>>
>> }
>>
>> If I set numTickets higher than 20, I get tons of timeout errors.
>>
>> I've also tried doing single commands with BatchStatement with many
>> inserts at a time, and that fails with timeout when the batch size gets
>> more than 20.
>>
>> Increasing the write request timeout in cassandra.yaml makes it time out
>> at slightly higher numbers of concurrent requests.
>>
>> With periodic I'm able to get about 38k writes / second, and with batch
>> I'm able to get about 14k / second.
>>
>> Any tips on what I should be doing to get group commitlog_sync to work
>> properly? I didn't expect to have to do anything other than change the
>> config.
>>
>>

Re: Trouble with using group commitlog_sync

[Bowen Song via user]

The default commitlog_sync_group_window is very long for SSDs. Try 
reduce it if you are using SSD-backed storage for the commit log. 10-15 
ms is a good starting point. You may also want to increase the value of 
concurrent_writes, consider at least double or quadruple it from the 
default. You'll need even higher write concurrency for longer 
commitlog_sync_group_window.



On 23/04/2024 19:26, Nathan Marz wrote:
"batch" mode works fine. I'm having trouble with "group" mode. The 
only config for that is "commitlog_sync_group_window", and I have that 
set to the default 1000ms.


On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user 
 wrote:


Why would you want to set commitlog_sync_batch_window to 1 second
long when commitlog_sync is set to batch mode? The documentation


on this says:

/This window should be kept short because the writer threads
will be unable to do extra work while waiting. You may need to
increase concurrent_writes for the same reason/

If you want to use batch mode, at least ensure
commitlog_sync_batch_window is reasonably short. The default is 2
millisecond.


On 23/04/2024 18:32, Nathan Marz wrote:

I'm doing some benchmarking of Cassandra on a single m6gd.large
instance. It works fine with periodic or batch commitlog_sync
options, but I'm having tons of issues when I change it to
"group". I have "commitlog_sync_group_window" set to 1000ms.

My client is doing writes like this (pseudocode):

Semaphore sem = new Semaphore(numTickets);
while(true) {

sem.acquire();
session.executeAsync(insert.bind(genUUIDStr(), genUUIDStr(),
genUUIDStr())
            .whenComplete((t, u) -> sem.release())

}

If I set numTickets higher than 20, I get tons of timeout errors.

I've also tried doing single commands with BatchStatement with
many inserts at a time, and that fails with timeout when the
batch size gets more than 20.

Increasing the write request timeout in cassandra.yaml makes it
time out at slightly higher numbers of concurrent requests.

With periodic I'm able to get about 38k writes / second, and with
batch I'm able to get about 14k / second.

Any tips on what I should be doing to get group commitlog_sync to
work properly? I didn't expect to have to do anything other than
change the config.

Re: Trouble with using group commitlog_sync

[Nathan Marz]

"batch" mode works fine. I'm having trouble with "group" mode. The only
config for that is "commitlog_sync_group_window", and I have that set to
the default 1000ms.

On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user <
user@cassandra.apache.org> wrote:

> Why would you want to set commitlog_sync_batch_window to 1 second long
> when commitlog_sync is set to batch mode? The documentation
> 
> on this says:
>
> *This window should be kept short because the writer threads will be
> unable to do extra work while waiting. You may need to increase
> concurrent_writes for the same reason*
>
> If you want to use batch mode, at least ensure commitlog_sync_batch_window
> is reasonably short. The default is 2 millisecond.
>
>
> On 23/04/2024 18:32, Nathan Marz wrote:
>
> I'm doing some benchmarking of Cassandra on a single m6gd.large instance.
> It works fine with periodic or batch commitlog_sync options, but I'm having
> tons of issues when I change it to "group". I have
> "commitlog_sync_group_window" set to 1000ms.
>
> My client is doing writes like this (pseudocode):
>
> Semaphore sem = new Semaphore(numTickets);
> while(true) {
>
> sem.acquire();
> session.executeAsync(insert.bind(genUUIDStr(), genUUIDStr(), genUUIDStr())
> .whenComplete((t, u) -> sem.release())
>
> }
>
> If I set numTickets higher than 20, I get tons of timeout errors.
>
> I've also tried doing single commands with BatchStatement with many
> inserts at a time, and that fails with timeout when the batch size gets
> more than 20.
>
> Increasing the write request timeout in cassandra.yaml makes it time out
> at slightly higher numbers of concurrent requests.
>
> With periodic I'm able to get about 38k writes / second, and with batch
> I'm able to get about 14k / second.
>
> Any tips on what I should be doing to get group commitlog_sync to work
> properly? I didn't expect to have to do anything other than change the
> config.
>
>

Re: Trouble with using group commitlog_sync

[Bowen Song via user]

Why would you want to set commitlog_sync_batch_window to 1 second long 
when commitlog_sync is set to batch mode? The documentation 
 
on this says:


   /This window should be kept short because the writer threads will be
   unable to do extra work while waiting. You may need to increase
   concurrent_writes for the same reason/

If you want to use batch mode, at least ensure 
commitlog_sync_batch_window is reasonably short. The default is 2 
millisecond.



On 23/04/2024 18:32, Nathan Marz wrote:
I'm doing some benchmarking of Cassandra on a single m6gd.large 
instance. It works fine with periodic or batch commitlog_sync options, 
but I'm having tons of issues when I change it to "group". I have 
"commitlog_sync_group_window" set to 1000ms.


My client is doing writes like this (pseudocode):

Semaphore sem = new Semaphore(numTickets);
while(true) {

sem.acquire();
session.executeAsync(insert.bind(genUUIDStr(), genUUIDStr(),
genUUIDStr())
            .whenComplete((t, u) -> sem.release())

}

If I set numTickets higher than 20, I get tons of timeout errors.

I've also tried doing single commands with BatchStatement with many 
inserts at a time, and that fails with timeout when the batch size 
gets more than 20.


Increasing the write request timeout in cassandra.yaml makes it time 
out at slightly higher numbers of concurrent requests.


With periodic I'm able to get about 38k writes / second, and with 
batch I'm able to get about 14k / second.


Any tips on what I should be doing to get group commitlog_sync to work 
properly? I didn't expect to have to do anything other than change the 
config.

Trouble with using group commitlog_sync

[Nathan Marz]

I'm doing some benchmarking of Cassandra on a single m6gd.large instance.
It works fine with periodic or batch commitlog_sync options, but I'm having
tons of issues when I change it to "group". I have
"commitlog_sync_group_window" set to 1000ms.

My client is doing writes like this (pseudocode):

Semaphore sem = new Semaphore(numTickets);
while(true) {

sem.acquire();
session.executeAsync(insert.bind(genUUIDStr(), genUUIDStr(), genUUIDStr())
.whenComplete((t, u) -> sem.release())

}

If I set numTickets higher than 20, I get tons of timeout errors.

I've also tried doing single commands with BatchStatement with many inserts
at a time, and that fails with timeout when the batch size gets more than
20.

Increasing the write request timeout in cassandra.yaml makes it time out at
slightly higher numbers of concurrent requests.

With periodic I'm able to get about 38k writes / second, and with batch I'm
able to get about 14k / second.

Any tips on what I should be doing to get group commitlog_sync to work
properly? I didn't expect to have to do anything other than change the
config.