Date: Fri, 7 Jul 2000 09:00:01 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Chris Richards <crichard-freebsd@wso.williams.edu> Cc: freebsd-arch@FreeBSD.ORG Subject: Re: Alterations to vops Message-ID: <200007071600.JAA00696@apollo.backplane.com> References: <Pine.BSF.4.05.10007062327020.68909-100000@login-1.eunet.no> <200007070143.SAA96248@apollo.backplane.com> <20000706184859.C25571@fw.wintelcom.net> <20000706223150.A24949@klapaucius.zer0.org> <20000707080934.I25571@fw.wintelcom.net> <20000707114411.A31035@wso.williams.edu>
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:On Fri, Jul 07, 2000 at 08:09:34AM -0700, Alfred Perlstein wrote:
:
:> The only problem is that for the "average case" you waste nearly
:> 3.5k per directory cached, so it's only really a win on systems
:> with large amounts of ram.
:
:In your estimation, how large is "large"? Of course each site should
:test to see what best suits its needs, but I'm looking for a rough
:idea.
:
:-chris
The number is also not correct. Sure, on a directory by directory
basis the wastage can be 3.5K. But if the page is cached in the
VM Page cache it will fall under the center-weighted LRU algorithm
which means that idle directory cache pages will be reused by the
system fairly quickly.
In contrast, (512 byte) directory blocks sitting the buffer cache's
MALLOC space are not governed by standard cache reuse mechanisms and
are not as reusable.
My personal feeling is that the simple fact that the pages become
governed by standard reuse algorithms when in the VM Page cache more
then compensates for the 'wasted' memory.
And what does 'memory waste' mean anyway? If the system performs
better then we shouldn't really care that 3.5K out of 4K is wasted
for a few seconds, verses 512 bytes 'not wasted' but also held in
the malloc cache 'forever' (except when the malloc cache gets saturated,
in which case you reuse the blocks but are screwed anyway because
the malloc cache is too small).
-Matt
Matthew Dillon
<dillon@backplane.com>
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