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@sss.pgh.pa.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
reportsof 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
usefulperformance gain to    * be had, spending time on that seems unprofitable for now.    */
 

--  Bruce Momjian   bruce@momjian.us EnterpriseDB    http://www.enterprisedb.com
 + If your life is a hard drive, Christ can be your backup. +