Subject: page reuse while accessing files sequentially
To: None <firstname.lastname@example.org>
From: Frank Kardel <Frank.Kardel@Acrys.COM>
Date: 06/04/2004 10:07:41
just a few thoughs with respect to vm page usage, i haven't checked whether
this has been discussed before, if it has: I am sorry.
When writing a large backup data is copied sequentially from a file to
some place else.
The vm system seemingly keeps these pages around just like any other
file pages. Thus
there is a constant need for fresh pages and thus all other file pages
are pushed out in
favor of the pages for the sequential file copy.
Would it make sense to categorize file pages with respect to their usage
access/random access ? Thus the file page sub class could be further
sequential pages and random access pages.
A file(vnode) would start out as sequential access vnode until a
decision can be made, that
access is random. Indications could be:
- the vnode is mmapped
- non sequential access
Pages allocated could be marked with the sequential/random information
of the vnode they
belong to. The sequential page pool could be limited separatly to keep
it from monopolizing
the file page pool. Even the executables (mmap property) could be
handled that way without
the need for an executable page class.
A related strategy seems the exist in Solaris 8, where sequential access
leads to a FIFO
replacement strategy. This might be dangerous though, as we did have
once a state where
our java vm was seemingly on the FIFO replacement strategy making work
very slow :-( .
This effect should be avoided.
Any comments ?