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Re: cygserver - Postgres Multiple connection Load Testing - Inifinte Loop
On Fri, Mar 24, 2017 at 06:11:01PM +0100, Corinna Vinschen wrote:
> - cygserver is using a defined number of threads in a thread pool for
> application requests. Every request is added to a request submission
> queue and handled by the next free thread in the pool.
> The default number of threads in the pool is 10. Each wait for a
> semaphore is blocking one thread. If more than the number of threads
> in the pool are supposed to wait on a semaphore the pool starves.
Interesting. I can confirm that, without updating software, "cygserver -r 40"
fixes both my self-contained test and my PostgreSQL test case. Folks can use
that workaround in released-version installations.
> So what I did now is to allow cygserver to raise the number of worker
> threads on demand. That is, if a request is in the queue and all
> worker threads are busy, just create a new one.
> There's no way yet to drop threads again, but this should be a minor
> problem in scenarions which really have a lot of contention.
Agreed. This is nicer.
> I pushed a patchset now, and uploaded new developer snapshots for
> testing to https://cygwin.com/snapshots/
> Please give it a try
Self-contained test case results look good:
cygwin-20170321.tar.xz "cygserver -r40": ok
cygwin-20170324.tar.xz "cygserver -r40": ok
cygwin-20170321.tar.xz "cygserver -r10": freezes (expected)
cygwin-20170324.tar.xz "cygserver -r10": ok; cygserver output concludes with
"cygserver: All threads busy, added one (now 21)".
I then tried my PostgreSQL test case ("pgbench -i -s 50" once to setup, then
"pgbench -S -j2 -c16 -T900 -P5" to test):
cygwin-20170321.tar.xz "cygserver -r40": ok for >3600s
cygwin-20170324.tar.xz "cygserver -r40": limited freeze in <1000s; no
cygwin-20170321.tar.xz "cygserver -r10": classic freeze in <1000s (expected)
cygwin-20170324.tar.xz "cygserver -r10": limited freeze in <1000s; no
cygserver output for most of the run, then output concluding with
"cygserver: All threads busy, added one (now 15)" just before the freeze
I call the cygwin-20170324 freezes "limited" because the symptoms differ from
the classic freeze I described upthread. "strace /bin/true" and "cat
/proc/sysvipc/sem" do not hang, but every PostgreSQL backend process is stuck
waiting on a synchronization primitive.
I can distill another self-contained test case for the limited freeze seen in
cygwin-20170324, but that make take awhile. I'm sending this early report so
you're aware of the possible regression in cygwin-20170324.
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