Re: Posix Shared Mem patch

On Thu, Jun 28, 2012 at 7:00 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Jun 27, 2012 at 9:44 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> On Wed, Jun 27, 2012 at 12:00 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>>> Would Posix shmem help with that at all?  Why did you choose not to
>>>> use the Posix API, anyway?
>>
>>> It seemed more complicated.  If we use the POSIX API, we've got to
>>> have code to find a non-colliding name for the shm, and we've got to
>>> arrange to clean it up at process exit.  Anonymous shm doesn't require
>>> a name and goes away automatically when it's no longer in use.
>>
>> I see.  Those are pretty good reasons ...
>
> So, should we do it this way?
>
> I did a little research and discovered that Linux 2.3.51 (released
> 3/11/2000) apparently returns EINVAL for MAP_SHARED|MAP_ANONYMOUS.
> That combination is documented to work beginning in Linux 2.4.0.  How
> worried should we be about people trying to run PostgreSQL 9.3 on
> pre-2.4 kernels?  If we want to worry about it, we could try mapping a
> one-page shared MAP_SHARED|MAP_ANONYMOUS segment first.  If that
> works, we could assume that we have a working MAP_SHARED|MAP_ANONYMOUS
> facility and try to allocate the whole segment plus a minimal sysv
> shm.  If the single page allocation fails with EINVAL, we could fall
> back to allocating the entire segment as sysv shm.

Do we really need a runtime check for that? Isn't a configure check
enough? If they *do* deploy postgresql 9.3 on something that old,
they're building from source anyway...

> A related question is - if we do this - should we enable it only on
> ports where we've verified that it works, or should we just turn it on
> everywhere and fix breakage if/when it's reported?  I lean toward the
> latter.

Depends on the amount of expected breakage, but I'd lean towards teh
later as well.

> If we find that there are platforms where (a) mmap is not supported or
> (b) MAP_SHARED|MAP_ANON works but has the wrong semantics, we could
> either shut off this optimization on those platforms by fiat, or we
> could test not only that the call succeeds, but that it works
> properly: create a one-page mapping and fork a child process; in the
> child, write to the mapping and exit; in the parent, wait for the
> child to exit and then test that we can read back the correct
> contents.  This would protect against a hypothetical system where the
> flags are accepted but fail to produce the correct behavior.  I'm
> inclined to think this is over-engineering in the absence of evidence
> that there are platforms that work this way.

Could we actually turn *that* into a configure test, or will that be
too complex?

--
 Magnus Hagander
 Me: http://www.hagander.net/
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