Re: clearing filesystem cache for I/O benchmarks

From: Benjamin Rutt (rutt.4+news_at_osu.edu)
Date: 07/27/04

  • Next message: Andi Kleen: "Re: large, spurious[?] TSC skews on AMD 760MPX boards" 
    To: linux-kernel@vger.kernel.org
Date:	Tue, 27 Jul 2004 13:25:17 -0400

    Andrew Morton <akpm@osdl.org> writes:

    > (Please don't remove people from the email recipient list when doing kernel
    > work.)

    Sorry, I'm reading via gmane and my newsreader doesn't make it
    straightforward to do so. But I'll do it manually for you.

    > However... If you write any amount of data to a file with O_DIRECT, that
    > will, as a side-effect, remove _all_ of that file's pagecache. In 2.4 as
    > well as 2.6. So you could scrub the pagecache by reading the first 4k then
    > writing it back with O_DIRECT.

    Thanks, that does work for ext3, very well. It's obvious that it
    clears kernel page cache and not controller/disk cache.

    >> A related question...if no posix_fadvise() advice has been given, does
    >> reading sequentially every byte of an 8GB file on a machine with <=
    >> 8GB of RAM guarantee that any page cache data that existed on the
    >> machine prior to the start of the 8GB read is now gone?
    >
    > It's not guaranteed that this will work - if the pages which you're trying
    > to evict were accessed multiple times then it may take more page
    > replacement to reliably shoot them down. But writing a 2xmemory file and
    > then deleting it will be a reasonably effective way of evicting most of
    > the other pagecache.

    OK thanks, I'll take on good faith that this is the best scheme in
    general. I was actually doing a somewhat different approach, reading
    through a 2x memory "dummy" file before accessing the real file, but
    based on your advice, I'll instead just create a 2x "dummy" file,
    fsync it, and then delete it.

    Thanks for the tips, 

    -- 
Benjamin Rutt
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