Re: RFC: design for new VM

[Matthew Dillon <dillon@apollo.backplane.com>]

:>     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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