CVS commit: src/sys/kern

[Thor Lancelot Simon <tls%netbsd.org@localhost>]

Module Name:    src
Committed By:   tls
Date:           Fri Jan  9 06:26:15 UTC 2004

Modified Files:
        src/sys/kern: vfs_bio.c

Log Message:
Change BUFCACHE (default hard limit on physmem consumption by metadata
cache) from 30% to 20%.  This seems to significantly smooth the oscillation
between "almost no memory available" and "UVM free target available" caused
by the current sudden, heavy backpressure on the metadata cache.  We should
revisit this again once the backpressure mechanism is better tuned; ideally,
the hard limit should almost never come into play, because the metadata
cache should gradually give back pages as buffers hit the AGE list and as
the page cache demands them, rather than giving back a big slug of pages
all at once when UVM decides it's in a hurry and fires off the page daemon.

Just how well this adjustment works is likely to vary significantly from
machine to machine depending on I/O mix, filesystem frag size, and total
memory.  However, 20% seems to be quite a bit better than 30% on several
systems I've tested and is, coincidentally, more than enough to cache
the entire metadata working set of the AnonCVS server with 100 clients,
which is a useful worst-case stake in the ground...


To generate a diff of this commit:
cvs rdiff -r1.105 -r1.106 src/sys/kern/vfs_bio.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.