|| ||Linus Torvalds <torvalds-AT-osdl.org>|
|| ||Greg Stark <gsstark-AT-mit.edu>|
|| ||Re: statfs() / statvfs() syscall ballsup...|
|| ||Sun, 12 Oct 2003 09:13:50 -0700 (PDT)|
|| ||Joel Becker <Joel.Becker-AT-oracle.com>,
Jamie Lokier <jamie-AT-shareable.org>,
Trond Myklebust <trond.myklebust-AT-fys.uio.no>,
Ulrich Drepper <drepper-AT-redhat.com>,
Linux Kernel <linux-kernel-AT-vger.kernel.org>|
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").
> 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.
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