I get an error about DB_BUFFER_SMALL being undefined in this patch.
Should it have the same #ifdefs wrapped around it as are currently in
dbpf-keyval.c? It is using ENOMEM if DB_BUFFER_SMALL isnt' defined.
-Phil
Phil Carns wrote:
Thanks Sam! We will give these patches a try and report back.
-Phil
Sam Lang wrote:
Hi Phil,
Attached mult.patch implements iterating over the dspace db using
DB_MULTIPLE_KEY. This may allow for the db get call to do larger
reads from your SAN. I was seeing slightly better performance with
local disk after creating 20K files in a fresh storage space. Doing
strace doesn't show fewer mmaps or larger reads though, so I'm not
sure how berkeley db pulls in its pages. Anyway, if it helps improve
performance for you guys, I can clean it up a bit and commit it. I
don't think anything uses dspace_iterate_handles besides that ledger
handle management code.
You can fiddle the MAX_NUM_VERIFY_HANDLE_COUNT value to set how many
handles to get at a time. Right now its set to 4096. Keep in mind
that this requires a much larger buffer allocated in
dbpf_dspace_iterate_handles_op_svc, since we have to get keys and
values, so essentially we do a get with a buffer that's 4096*(sizeof
(handle) + sizeof(stored_attr)), which ends up being about 300K.
I also attached a patch (server-start.patch) that prints out the
start message as well as ready message after server initialization
has completed. If you set the Logstamp to usec, you'll be able to
see the time it takes to initialize the server. Also, this might
help in knowing when you can mount the clients, although, hopefully
at some point we'll be able to add the zero-conf stuff and then we
can return EAGAIN or something.
I'm not sure its time to replace the ledger code. It seems to work
ok, and to fix the slowness you're seeing would mean switching to
some kind of range tree that could be serialized to disk so that we
wouldn't have to iterate through the entire dspace db on startup.
That opens up the possibility of the dspace db and the ledger-on-disk
getting out of sync, which I'd rather avoid.
We could hand out new handles by choosing one randomly, and then
checking if its in the DB, getting rid of the need for a ledger
entirely, but I assume this idea was already scratched to avoid the
potential costs at creation time, especially as the filesystem grows.
-sam
On Feb 20, 2007, at 11:23 AM, Phil Carns wrote:
Robert Latham wrote:
On Tue, Feb 20, 2007 at 07:29:16AM -0500, Phil Carns wrote:
Oh, and one other detail; the memory usage of the servers looks
fine during startup, so this doesn't appear to be a memory leak.
There is quite a bit of CPU work, but I am guessing that is just
berkeley db keeping busy in the iteration function.
How long does it take to scan 1.4 million files on startup?
==rob
That's an interesting issue :)
A few observations:
- we were looking at this on SAN; the results may be different on
local disks
- the db files are on the order of 500 MB for this particular setup
- the time to scan varies depending on if the db files are hot in
the Linux buffer cache
If we start the daemon right after killing another one that just did
the same scan, then the process is CPU intensive, but fast (about 5
seconds). If we unmount/mount the SAN between the two runs so that
the buffer cache is cleared, then it is very slow (about 5 minutes).
An interesting trick is to use dd with a healthy buffer size to read
the .db files and throw the output into /dev/null before starting
the servers. This only takes a few seconds, and makes it so that
the scan consistently finishes in just a few seconds as well. I
think the reason is just that it forces the db data into the Linux
buffer cache using an efficient access pattern so that berkeley db
doesn't have to wait on disk latency for whatever small accesses it
is performing.
This seems to indicate that berkeley db's access pattern generated
by PVFS2 for this case isn't very friendly, at least to SANs that
aren't specifically tuned for it.
The 5 minute scan time is a problem, because it makes it hard to
tell when you will actually be able to mount the file system after
the daemons appear to have started. We would be happy to try out
any optimizations here :)
-Phil
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