Re: statfs() / statvfs() syscall ballsup...

From: Linus Torvalds (torvalds_at_osdl.org)
Date: 10/12/03

  • Next message: Tony Lindgren: "[PATCH] Make pmdisk suspend more reliable" 
    Date:	Sun, 12 Oct 2003 09:13:50 -0700 (PDT)
To: Greg Stark <gsstark@mit.edu>

    On 12 Oct 2003, Greg Stark wrote:
    >
    > There are other reasons databases want to control their own cache. The
    > application knows more about the usage and the future usage of the data than
    > the kernel does.

    But this again is not an argument for not using the page cache - it's only
    an argument for _telling_ the kernel about its use.

    > However on busy servers whenever it's run it causes lots of pain because the
    > kernel flushes all the cached data in favour of the data this job touches.

    Yes. But this is actually pretty easy to avoid in-kernel, since all of the
    LRU logic is pretty localized.

    It could be done on a per-process thing ("this process should not pollute
    the active list") or on a per-fd thing ("accesses through this particular
    open are not to pollute the active list").

    > And
    > worse, there's no way to indicate that the i/o it's doing is lower priority,
    > so i/o bound servers get hit dramatically.

    IO priorities are pretty much worthless. It doesn't _matter_ if other
    processes get preferred treatment - what is costly is the latency cost of
    seeking. What you want is not priorities, but batching.

                            Linus

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  • Next message: Tony Lindgren: "[PATCH] Make pmdisk suspend more reliable"

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