cygserver - Postgres Multiple connection Load Testing - Inifinte Loop

Noah Misch
Tue Mar 28 05:48:00 GMT 2017

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

> 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
  cygserver output
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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