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SubjectRe: Page cache patch - 2'nd version
Ekkk!  No!

This changes the ageing of named (mapped) pages to be very aggresive,
which will result in many more page faults and file-system read/writes.
The faults may be hard (that is all mappings will have been undone,
and the page removed from the cache, so it will have to be re-read from
store), or soft (that is the page has survived reaping, and is still in
the page-cache, but this is still expensive).
The only guard left against reaping named (mapped) pages will be the
'young' attribute of the PTE.

Yes, it will mean that named (mapped) pages can be reaped easier, which
for some usage patterns will be a win (mainly those pages with a short
time in a working set. A reasonable example is a compiling a kernel).
And it will save CPU cycles in swap_out() - less deadend searches.

However, if you compile a kernel on a box where users are (say) using
bash/vi, they will quickly find their working set of named pages reduced
(and quickly reduced again, if it does happen to grow). This is not very
friendly.

If you want pages to be reaped easier, use the page-ageing tunables!

NOTE: Named pages are given an initial age of PAGE_AGE_VALUE (see
pagemap.h - this is not a soft-tunable), which is 16. The configured
default age for anonymous (and shm()) pages is PAGE_INITIAL_AGE, which is
20.

If swap is configured as a device, as opposed to a file, it is faster to
read/write an anonymous-page to/from swap than a named-page to/from the
file-system (there is less code overhead). So there should be a slight
preference to reap anonymous pages (they should be given a smaller initial
page age), but in the current implementation this is not true.
Why? The swap-device does not perform readahead. To do so, 'related'
anonymous pages (that is pages from the same vm_area) would need to be
written down in clusters - treated as a large page _when_ it makes sense.

Memory management does need to be made more flexible in Linux. There
are already a few patches around. Unfortunately, to go much further MM
needs a re-work.

Regards,

markhe

------------------------------------------------------------------
Mark Hemment, Unix/C Software Engineer (Contractor)
markhe@nextd.demon.co.uk http://www.nextd.demon.co.uk/
"Success has many fathers, failure is a B**TARD!" - anon
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