From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Matthew Kirkwood <matthew(at)hairy(dot)beasts(dot)org> |
Cc: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org |
Subject: | mmap for zeroing WAL log |
Date: | 2001-02-25 04:28:53 |
Message-ID: | 19434.983075333@sss.pgh.pa.us |
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Matthew Kirkwood <matthew(at)hairy(dot)beasts(dot)org> writes:
> On Sat, 24 Feb 2001, Bruce Momjian wrote:
>> I am confused why mmap() is better than writing to a real file.
> It isn't, except that it allows to initialise the logfile in
> one syscall, without first allocating and zeroing (and hence
> dirtying) 16Mb of memory.
Uh, the existing code does not zero 16Mb of memory... it zeroes
8K and then writes that block repeatedly. It's possible that the
overhead of a syscall for each 8K block is significant, but on the
other hand writing a block at a time is a heavily used and heavily
optimized path in all Unixen. It's at least as plausible that the
mmap-as-source-of-zeroes path will be slower!
I think this is worth looking into, but I'm very far from being
sold on it...
regards, tom lane
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