Re: double-buffering page writes

From: ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: double-buffering page writes
Date: 2008-10-23 02:09:43
Message-ID: 20081023105338.B715.52131E4D@oss.ntt.co.jp
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Alvaro Herrera <alvherre(at)commandprompt(dot)com> wrote:

> I'm trying to see if it makes sense to do the double-buffering of page
> writes before going further ahead with CRC checking. I came up with the
> attached patch; it does the double-buffering inconditionally, because as
> it was said, it allows releasing the io_in_progress lock (and resetting
> BM_IO_IN_PROGRESS) early.

I have some comments about the double-buffering:

- Are there any performance degradation because of addtional memcpy?
8kB of memcpy seems not to be free.

- Is it ok to allocale dblbuf[BLCKSZ] as local variable?
It might be unaligned. AFAICS we avoid such usages in other places.

- It is the best if we can delay double-buffering until locks are
conflicted actually. But we might need to allocale shadow buffers
from shared buffers instead of local memory.

- Are there any other modules that can share in the benefits of
double-buffering? For example, we could avoid avoid waiting for
LockBufferForCleanup(). It is cool if the double-buffering can
be used for multiple purposes.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center

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