From: | Manfred Spraul <manfred(at)colorfullife(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | pgsql-patches(at)postgresql(dot)org |
Subject: | Re: performance: use pread instead of lseek+read |
Date: | 2003-02-25 06:35:13 |
Message-ID: | 3E5B0EA1.6050805@colorfullife.com |
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Lists: | pgsql-patches |
Tom Lane wrote:
>Manfred Spraul <manfred(at)colorfullife(dot)com> writes:
>
>
>>Tom Lane wrote:
>>
>>
>>>It seems unlikely to me that eliminating lseek on some platforms would
>>>be worth the hassle of maintaining two code paths. lseek is mighty
>>>cheap as system calls go.
>>>
>>>
>>>
>>It was considered expensive enough to write a syscall avoidance layer
>>that caches the file pointer and skips lseek if fpos==offset.
>>
>>
>
>You're missing the point: that layer is mostly there to ensure that we
>don't foul up the kernel's readahead recognition for sequential fetches.
>It's nice that Linux doesn't care, but Linux is not the only platform
>we worry about.
>
>
Do you know that empty lseeks foul up readahead recognition on some OS?
If yes, which OS? I've checked FreeBSD and Linux, they don't do it.
Actually I would be really surprised if pread would cause readahead
problems - for example samba uses it if possible.
What about my other questions:
- which benchmark would be interesting?
- which OS did you use when you got 'no manpage for pread'?
--
Manfred
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