Re: [PATCH -mm] mm: more likely reclaim MADV_SEQUENTIAL mappings

On Tuesday 22 July 2008 13:43, Nick Piggin wrote:
On Tuesday 22 July 2008 13:04, Rik van Riel wrote:
On Tue, 22 Jul 2008 12:54:28 +1000

But we are not doing nothing because we already know and have coded
for the fact that the mapping will be accessed once, sequentially.
Now that we have gone this far, we should actually do it properly and
1. unmap after use, 2. POSIX_FADV_DONTNEED after use. This will give
you much better performance and cache behaviour than any automatic
detection scheme, and it doesn't introduce any regressions for existing
code.

If you run just one instance of the application!

Think about something like an ftp server or a media server,
where you want to cache the data that is served up many
times, while evicting the data that got served just once.

The kernel has much better knowledge of what the aggregate
of all processes in the system are doing than any individual
process has.

That's true, but this case isn't really very good anyway. The information
goes away after you drop the mapping anyway. Or did you hope that the
backup program or indexer keeps all those mappings open until all the pages
have filtered through? Or maybe we can add yet more branches into the unmap
path to test for this flag as well?

I don't think it is a good idea to add random things just because they seem
at first glance like a good idea.

BTW. in the backup of a busy fileserver or some case like that, I'd
bet that even using FADV_DONTNEED would be much faster than leaving
these mappings around to try to drop them due to the decreased churn
on the LRUs overall anyway.

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