Re: [SPAM?] Re: Asynchronous I/O Support

Martijn van Oosterhout wrote:
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> On Fri, Oct 20, 2006 at 03:04:55PM -0400, Merlin Moncure wrote:
> > On 10/20/06, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > >So far I've seen no evidence that async I/O would help us, only a lot
> > >of wishful thinking.
> >
> > is this thread moot? while researching this thread I came across this
> > article: http://kerneltrap.org/node/6642 describing claims of 30%
> > performance boost when using posix_fadvise to ask the o/s to prefetch
> > data. istm that this kind of improvement is in line with what aio can
> > provide, and posix_fadvise is cleaner, not requiring threads and such.
>
> Hmm, my man page says:
>
> POSIX_FADV_WILLNEED and POSIX_FADV_NOREUSE both initiate a
> non-blocking read of the specified region into the page cache.
> The amount of data read may be decreased by the kernel depending
> on VM load. (A few megabytes will usually be fully satisfied,
> and more is rarely useful.)
>
> This appears to be exactly what we want, no? It would be nice to get
> some idea of what systems support this.

See our xlog.c for our experience in trying to use it:

/*
* posix_fadvise is problematic on many platforms: on older x86 Linux it
* just dumps core, and there are reports of problems on PPC platforms as
* well. The following is therefore disabled for the time being. We could
* consider some kind of configure test to see if it's safe to use, but
* since we lack hard evidence that there's any useful performance gain to
* be had, spending time on that seems unprofitable for now.
*/

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Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

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