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Re: posix_fadvise v22

From: Greg Stark <greg(dot)stark(at)enterprisedb(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Gregory Stark <stark(at)enterprisedb(dot)com>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, Postgres <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: posix_fadvise v22
Date: 2009-01-02 03:53:54
Message-ID: 9DCB7B9A-50E0-477E-972F-01754B3EE85F@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
In theory there should be no benefit on a single spindle system. There  
could be a slight benefit due to reordering of I/o but only on a raid  
array would you see a significant speedup -- which should be about  
equal to the number of spindles.

What would be interesting is whether you see a noticable speed  
*decrease* from having prefetching enabled when it isn't helping.  
Either due to having everything fit in shared buffers or everything  
fit in the filesystem cache (the latter should be more of a hit)

Even if there is it doesn't really worry me. By default the feature is  
disabled and you should only really turn it on if ulu do have a raid  
array and want an individual query to make use if it.


Now that there's an actual run-time sysconf check for the buggy glibc  
called by the guc function we arguably don't need the autoconf  
check_run check anymore anyways.

-- 
Greg


On 1 Jan 2009, at 21:43, "Robert Haas" <robertmhaas(at)gmail(dot)com> wrote:

> On Thu, Jan 1, 2009 at 3:55 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> "Robert Haas" <robertmhaas(at)gmail(dot)com> writes:
>>> Am I correct in thinking that the only thing we're really checking  
>>> for
>>> here is whether a trivial posix_fadvise() call returns success?  If
>>> so, is this test really worth doing?
>>
>> Runtime tests performed during configure are generally a bad idea to
>> start with --- it's impossible to do any such thing in a
>> cross-compilation scenario, for example.
>
> OK, here's an update of Greg's patch with the runtime configure test
> ripped out, some minor documentation tweaks, and a few unnecessary
> whitespace diff hunks quashed.  I think this is about ready for
> committer review.  The only thing I haven't been able to do is
> demonstrate that this change actually produces a performance
> improvement.  Either I'm testing the wrong thing, or it just doesn't
> provide any benefit on a single-spindle system.  However, I believe
> that Greg has previously posted some fairly impressive performance
> results, so I'm not sure that my shortcomings in this area should be a
> bar to having a committer pick this one up.  If more testing is
> needed, it would at least be helpful to have a committer specify what
> areas they are concerned about.
>
> ...Robert
> <posix_fadvise_v23_rh1.diff.gz>

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