Messages in this thread | | | Date | Mon, 26 Jul 2004 23:40:05 -0700 | From | Andrew Morton <> | Subject | Re: clearing filesystem cache for I/O benchmarks |
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(Please don't remove people from the email recipient list when doing kernel work.)
Benjamin Rutt <rutt.4+news@osu.edu> wrote: > > Andrew Morton <akpm@osdl.org> writes: > > > Benjamin Rutt <rutt.4+news@osu.edu> 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.
OK.
> 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.
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