Re: [PATCH] fs: add fincore(2) (mincore(2) for file descriptors)

[Andy Isaacson <adi@hexapodia.org>]

On Tue, Feb 16, 2010 at 10:13:12AM -0800, Chris Frost wrote:
> Add the fincore() system call. fincore() is mincore() for file descriptors.
> 
> The functionality of fincore() can be emulated with an mmap(), mincore(),
> and munmap(), but this emulation requires more system calls and requires
> page table modifications. fincore() can provide a significant performance
> improvement for non-sequential in-core queries.

In addition to being expensive, mmap/mincore/munmap perturb the VM's
eviction algorithm -- a page is less likely to be evicted if it's
mmapped when being considered for eviction.

I frequently see this happen when using mincore(1) from
http://bitbucket.org/radii/mincore/ -- "watch mincore -v *.big" while
*.big are being sequentially read results in a significant number of
pages remaining in-core, whereas if I only run mincore after the
sequential read is complete, the large files will be nearly-completely
out of core (except for the tail of the last file, of course).

It's very interesting to watch
% watch --interval=.5 mincore -v *

while an IO-intensive process is happening, such as mke2fs on a
filesystem image.

So, I support the addition of fincore(2) and would use it if it were
merged.

-andy