Re: O_DIRECT question

[Nick Piggin]

Aubrey wrote:
> On 1/11/07, Roy Huang <royhuang9@xxxxxxxxx> wrote:
>
> > On a embedded systerm, limiting page cache can relieve memory
> > fragmentation. There is a patch against 2.6.19, which limit every
> > opened file page cache and total pagecache. When the limit reach, it
> > will release the page cache overrun the limit.
>
>
> The patch seems to work for me. But some suggestions in my mind:
>
> 1) Can we limit the total page cache, not the page cache per each file?
>    think about if total memory is 128M, 10% of it is 12.8M, here if
> one application is running, it can use 12.8M vfs cache, then the
> performance will probably not be impacted. However, the current patch
> limit the page cache per each file, which means if only one
> application runs it can only use CONFIG_PAGE_LIMIT pages cache. It may
> be small to the application.
> ------------------snip---------------
> if (mapping->nrpages >= mapping->pages_limit)
>               balance_cache(mapping);
> ------------------snip---------------
>
> 2) A percent number should be better to control the value. Can we add
> a proc interface to make the value tunable?

Even a global value isn't completely straightforward, and a per-file value
would be yet more work.

You see, it is hard to do any sort of directed reclaim at these pages.

--
SUSE Labs, Novell Inc.
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