[lkml]   [2004]   [Jul]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: clearing filesystem cache for I/O benchmarks

(Please don't remove people from the email recipient list when doing kernel

Benjamin Rutt <> wrote:
> Andrew Morton <> writes:
> > Benjamin Rutt <> wrote:
> >>
> >> How can I purge all of the kernel's filesystem caches, so I can trust
> >> that my I/O (read) requests I'm trying to benchmark bypass the kernel
> >> filesystem cache?
> >
> > Either delete the benchmark test files or
> I'm not sure I follow. If I delete the benchmark files, I'll only
> need to create them again later in order to do a read test, and I'll
> have the same problem then, of how to eliminate the just-written-data
> from cache.


> Thanks for the reference, I wasn't aware of that one. We are running
> some 2.4 kernels in our storage cluster unfortunately so that won't be
> usable for us everywhere. I take it POSIX_FADV_DONTNEED is ignored
> under 2.4.

posix_fadvise() will return -ENOSYS under 2.4.

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.

However O_DIRECT is supported on very few filesystems in 2.4. ext2 and
reiserfs have it.

XFS in 2.4 has O_DIRECT, I think, but I don't know if the invalidation
side-effect works on XFS.

> 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.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-03-22 14:04    [W:0.094 / U:0.100 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site