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Message-ID: <7be3f35d0610160518h649c8f20pee9949a90796c346@mail.gmail.com>
Date: Mon, 16 Oct 2006 14:18:30 +0200
From: "Harald Armin Massa" <haraldarminmassa@gmail.com>
To: "Magnus Hagander" <mha@sollentuna.net>
Subject: Re: measuring shared memory usage on Windows
Cc: pgsql-performance@postgresql.org
In-Reply-To: <6BCB9D8A16AC4241919521715F4D8BCEA357F4@algol.sollentuna.se>
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References: <7be3f35d0610160418u51e4d256k15ee2a5e4ba9433@mail.gmail.com>
	<6BCB9D8A16AC4241919521715F4D8BCEA357F4@algol.sollentuna.se>

Magnus,

> > "anonymous mapped memory"  site:microsoft.com
> > turns out 0 (zero) results. And even splitting it up there
> > seems to be nearly no information ... is the same thing by
> > any chance also known by different names?
>
> Hmm. Yeah, most likely :) I may have grabbed that name from something
> else. THe documentation for the call is on
> http://windowssdk.msdn.microsoft.com/en-us/library/ms685007(VS.80).aspx,
> we specify INVALID_HANDLE_VALUE for hFile, which means:

[...]
CreateFileMapping creates a file mapping object of a specified size
that _the operating system paging file backs_
[...]

I assume that DWORD dwMaximumSizeHigh and  DWORD dwMaximumSizeLow
get filled with whatever I configure in shared_memory?

My reading of that function gives me the impression, that this kind of
shared *memory* is essentially a shared disk file - "_the operating
system paging file backs_"

Especially documentation lines like "If an application specifies a
size for the file mapping object that is larger than the size of the
actual named file on disk, the file on disk is increased to match the
specified size of the file mapping object."

really makes me think that that area is just a comfortable way to
access files on disk as memory areas; with the hope of propably better
caching then not-memory-mapped files.

That would explain my disturbing impressions of performance of
PostgreSQL on win32 rising when lowering shared_memory...

Harald
--=20
GHUM Harald Massa
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Harald Armin Massa
Reinsburgstra=DFe 202b
70197 Stuttgart
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