Horton

Re: "could not reattach to shared memory" on buildfarm member dory

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Noah Misch <noah@leadboat.com>
Cc: Heath Lord <heath.lord@crunchydata.com>, Stephen Frost <sfrost@snowman.net>, Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, joseph.ayers@crunchydata.com
Date: 2019-04-02T14:09:00Z
Lists: pgsql-hackers 
Noah Misch <noah@leadboat.com> writes:
> I can reproduce the 4 MiB allocations described
> in https://postgr.es/m/29823.1525132900@sss.pgh.pa.us; a few times per
> "vcregress check", they emerge in the middle of PGSharedMemoryReAttach().
> The 4 MiB allocations are stacks for new threads of the default thread
> pool[1].

Hah!  It is great to finally have an understanding of what is happening
here.

I worry that your proposed fix is unstable, in particular this assumption
seems shaky:

> + * ... The idea is that, if the allocator handed out
> + * REGION1 pages before REGION2 pages at one occasion, it will do so whenever
> + * both regions are free.

I wonder whether it's possible to address this by configuring the "default
thread pool" to have only one thread?  It seems like the extra threads are
just adding backend startup overhead to no benefit, since we won't use 'em.

			regards, tom lane

Commits

  1. Avoid "could not reattach" by providing space for concurrent allocation.

  2. Assert that pgwin32_signal_initialize() has been called early enough.

  3. Remove investigative code for can't-reattach-to-shared-memory errors.

  4. Does it help to wait before reattaching?

  5. Map and unmap the shared memory block before risking VirtualFree.

  6. Further effort at preventing memory map dump from affecting the results.

  7. Remove Windows module-list-dumping code.

  8. Dump full memory maps around failing Windows reattach code.

  9. Get still more info about Windows can't-reattach-to-shared-memory errors.

  10. Get more info about Windows can't-reattach-to-shared-memory errors.

  11. Try to get some info about Windows can't-reattach-to-shared-memory errors.