CVS commit: src/sys/rump/librump/rumpkern

[Antti Kantee <pooka%netbsd.org@localhost>]

Module Name:    src
Committed By:   pooka
Date:           Fri May 28 16:44:14 UTC 2010

Modified Files:
        src/sys/rump/librump/rumpkern: rump.c scheduler.c threads.c

Log Message:
Improve the CPU scheduler for a host MP system with multithreaded
access.  The old scheduler had a global freelist which caused a
cache crisis with multiple host threads trying to schedule a virtual
CPU simultaneously.

The rump scheduler is different from a normal thread scheduler, so
it has different requirements.  First, we schedule a CPU for a
thread (which we get from the host scheduler) instead of scheduling
a thread onto a CPU.  Second, scheduling points are at every
entry/exit to/from the rump kernel, including (but not limited to)
syscall entry points and hypercalls.  This means scheduling happens
a lot more frequently than in a normal kernel.

For every lwp, cache the previously used CPU.  When scheduling,
attempt to reuse the same CPU.  If we get it, we can use it directly
without any memory barriers or expensive locks.  If the CPU is
taken, migrate.  Use a lock/wait only in the slowpath.  Be very
wary of walking the entire CPU array because that does not lead to
a happy cacher.

The migration algorithm could probably benefit from improved
heuristics and tuning.  Even as such, with the new scheduler an
application which has two threads making rlimit syscalls in a tight
loop experiences almost 400% speedup.  The exact speedup is difficult
to pinpoint, though, since the old scheduler caused very jittery
results due to cache contention.  Also, the rump version is now
70% faster than the counterpart which calls the host kernel.


To generate a diff of this commit:
cvs rdiff -u -r1.171 -r1.172 src/sys/rump/librump/rumpkern/rump.c
cvs rdiff -u -r1.14 -r1.15 src/sys/rump/librump/rumpkern/scheduler.c
cvs rdiff -u -r1.8 -r1.9 src/sys/rump/librump/rumpkern/threads.c

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