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SubjectRe: RFC: design for new VM

:> if 300 processes fault on the same backing file offset you are going
:> to hit a bottleneck with MP locking anyway, just at a deeper level
:> (the filesystem rather then the VM system).
:
:Erm... I'm not sure about that - for one thing, you are not caching
:results of bmap(). We do. And our VFS is BKL-free, so contention really
:hits only on the VOP_BALLOC() level (that can be fixed too, but that's
:another story).

Well... actually, a side effect of the FreeBSD buffer cache is to
cache BMAP translations.

What we do do, which kinda kills the cacheability aspects of balloc
in some cases, is VOP_REALLOC() -- that is, the FFS filesystem will
reallocate blocks to implement on-the-fly defragmentation. This
typically occurs on writes. It works *very* well. This is a feature
that actually used to be in FFS a few years ago but had to be turned off
due to bugs. The bugs were fixed about a year ago and realloc was turned
on by default in 4.x.

-Matt
Matthew Dillon
<dillon@backplane.com>

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