CVS commit: src/sys/kern

[Andrew Doran <ad%netbsd.org@localhost>]

Module Name:    src
Committed By:   ad
Date:           Fri Apr  4 19:16:24 UTC 2008

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

Log Message:
When a timeshared LWP blocks on a turnstile, elevate its priority into the
PRI_KTHREAD range. This is kind of ugly, but needed because of direct handoff
with rwlocks, and because threads that block holding a mutex regularly hold
other locks/resources.

Problem addressed: priority lending works well where a thread blocking on a
turnstile has a high priority level (eg realtime). For timeshared threads
(low priority) it's unlikely to have much effect. In the latter case threads
awoken from a turnstile can and do compete for CPU time with regular waits
like disk I/O. On MP systems this can result in a feedback loop where
threads cannot quickly get access to a resource held by a thread waking from
a turnstile. The waking thread eventually runs when enough of the other
threads block waiting for it, freeing up the CPU. The end result is a lot of
idle time during builds.


To generate a diff of this commit:
cvs rdiff -r1.16 -r1.17 src/sys/kern/kern_turnstile.c

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