Thanks for the suggestions Aaron.
As a follow up, we ran a bunch of tests with different combinations of these
changes on a 2-node ring. The load was generated using cassandra-stress, run
with default values to write 30 million rows, and read them back.
However, for both writes and reads there was virtually no difference in the
latencies.
The different combinations attempted:
1. Baseline test with none of the below changes.
2. Grabbing the TLAB setting from 1.2
3. Moving the commit logs too to the 7 disk RAID 0.
4. Increasing the concurrent_read to 32, and concurrent_write to 64
5. (3) + (4), i.e. moving commit logs to the RAID + increasing
concurrent_read and concurrent_write config to 32 and 64.
The write latencies were very similar, except them being ~3x worse for the
99.9th percentile and above for scenario (5) above.
The read latencies were also similar, with (3) and (5) being a little worse for
the 99.99th percentile.
Overall, not making any changes, i.e. (1) performed as well or slightly better
than any of the other changes.
Running cassandra-stress on both the old and new hardware without making any
config changes, the write performance was very similar, but the new hardware
did show ~10x improvement in the read for the 99.9th percentile and higher.
After thinking about this, the reason why we were not seeing any difference
with our test framework was perhaps the nature of the test where we write the
rows, and then do a bunch of reads to read the rows that were just written
immediately following. The data is read back from the memtables, and never from
the disk/sstables. Hence the new hardware's increased RAM and size of the disk
cache or higher number of disks never helps.
I'm still not very sure where the current *write* bottleneck is though. The new
hardware has 32 cores vs 8 cores of the old hardware. Moving the commit log
from a dedicated disk to a 7 RAID-0 disk system (where it would be shared by
other data though) didn't make a difference too. (unless the extra contention
on the RAID nullified the positive effects of the RAID).
Sample iostat data (captured every 10s) for the dedicated disk where commit
logs are written is below. Does this seem like a bottle neck? When the commit
logs are written the await/svctm ratio is high.
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz
await svctm %util
0.00 8.09 0.04 8.85 0.00 0.07 15.74 0.00
0.12 0.03 0.02
0.00 768.03 0.00 9.49 0.00 3.04 655.41 0.04
4.52 0.33 0.31
0.00 8.10 0.04 8.85 0.00 0.07 15.75 0.00
0.12 0.03 0.02
0.00 752.65 0.00 10.09 0.00 2.98 604.75 0.03
3.00 0.26 0.26
Another interesting thing is that the linux disk cache doesn't seem to be
growing in spite of a lot of free memory available. The total disk cache used
reported by 'free' is less than the size of the sstables written with over 100
GB unused RAM.
Even in production, where we have the older hardware running with 32 GB RAM for
a long time now, looking at 5 hosts in 1 DC, only 2.5 GB to 8 GB was used for
the disk cache. The Cassandra java process uses the 8 GB allocated to it, and
at least 10-15 GB on all the hosts is not used at all.
Thanks,
Arindam
From: Aaron Morton [mailto:[email protected]]
Sent: Wednesday, November 06, 2013 8:34 PM
To: Cassandra User
Subject: Re: Config changes to leverage new hardware
Running Cassandra 1.1.5 currently, but evaluating to upgrade to 1.2.11 soon.
You will make more use of the extra memory moving to 1.2 as it moves bloom
filters and compression data off heap.
Also grab the TLAB setting from cassandra-env.sh in v1.2
As of now, our performance tests (our application specific as well as
cassandra-stress) are not showing any significant difference in the hardwares,
which is a little disheartening, since the new hardware has a lot more RAM and
CPU.
For reads or writes or both ?
Writes tend to scale with cores as long as the commit log can keep up.
Reads improve with disk IO and page cache size when the hot set is in memory.
Old Hardware: 8 cores (2 quad core), 32 GB RAM, four 1-TB disks ( 1 disk used
for commitlog and 3 disks RAID 0 for data)
New Hardware: 32 cores (2 8-core with hyperthreading), 128 GB RAM, eight 1-TB
disks ( 1 disk used for commitlog and 7 disks RAID 0 for data)
Is the disk IO on the commit log volume keeping up ?
You cranked up the concurrent writers and the commit log may not keep up. You
could put the commit log on the same RAID volume to see if that improves writes.
The config we tried modifying so far was concurrent_reads to (16 * number of
drives) and concurrent_writes to (8 * number of cores) as per
256 write threads is a lot. Make sure the commit log can keep up, I would put
it back to 32, maybe try 64. Not sure the concurrent list for the commit log
will work well with that many threads.
May want to put the reads down as well.
It's easier to tune the system if you can provide some info on the workload.
Cheers
-----------------
Aaron Morton
New Zealand
@aaronmorton
Co-Founder & Principal Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com
On 7/11/2013, at 12:35 pm, Arindam Barua
<[email protected]<mailto:[email protected]>> wrote:
We want to upgrade our Cassandra cluster to have newer hardware, and were
wondering if anyone has suggestions on Cassandra or linux config changes that
will prove to be beneficial.
As of now, our performance tests (our application specific as well as
cassandra-stress) are not showing any significant difference in the hardwares,
which is a little disheartening, since the new hardware has a lot more RAM and
CPU.
Old Hardware: 8 cores (2 quad core), 32 GB RAM, four 1-TB disks ( 1 disk used
for commitlog and 3 disks RAID 0 for data)
New Hardware: 32 cores (2 8-core with hyperthreading), 128 GB RAM, eight 1-TB
disks ( 1 disk used for commitlog and 7 disks RAID 0 for data)
Most of the cassandra config currently is the default, and we are using
LeveledCompaction strategy. Default key cache, row cache turned off.
The config we tried modifying so far was concurrent_reads to (16 * number of
drives) and concurrent_writes to (8 * number of cores) as per recommendation in
cassandra.yaml, but that didn't make much difference.
We were hoping that at least the extra RAM in the new hardware will be used for
Linux file caching and hence an improvement in performance will be observed.
Running Cassandra 1.1.5 currently, but evaluating to upgrade to 1.2.11 soon.
Thanks,
Arindam
