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[20090112]
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More kernel tuning in progress
Andrew Doran is at it again, and he has proposed a number
of patches to further improve NetBSD's performance in
various areas:
- Optimization for exec by using a cached copy of
the file's exec header, and reducing locking-overhead by keeping
locks instead of freeing them and the immediately re-locking them.
- The NetBSD kernel offers a few internal interfaces for allocating
memory, depending on use of the memory, duration of use, size,
etc. For some of the proper SMP-handling is important, and thus
the number of CPUs has an impact on their performance.
By adding caching,
this can be mitigated to get linear behaviour, independent
from the number of CPUs:
- The openat() and fstatat() system calls are not available in NetBSD
yet, but other systems offer them, standards are about to pick
them up, and ZFS assumes their presence. The system calls offer a way
to way to specify a different directory to which relative paths
are relative to, other than ".", by passing a file descriptor for
that directory. Now there is a patch to
add openat() and fstatat() to NetBSD.
- In theory, pipes are just a special case of (host-local) sockets,
but it makes sense to keep a separate implementation for reasons of
speed optimizations. NetBSD has the "PIPE_SOCKETPAIR" kernel option
to force use of the socket code for pipes in order to reduce
the memory footprint, but
benchmarks reflect the performace hit.
In order to improve the the situation,
a number of improvements are under way,
including better cache utilization and SMP-compliant memory
allocation over homegrown memory management.
- Improved performance of exit(2)
- this is important in environments with many short-running processes
(think httpd, inetd).
- As a final step, freeing entries in the translation
lookaside buffer (TLB) of the x86 (i386, amd64, xen) memory management unit
AKA TLB shootdown
were sped up
to a point where TLB shootdown interrupts are 50% down during a system
rebuild on an 8-cpu machine, and several million(!) calls page
invalidation were optimized away, resulting in a 1% speed increase
on the overall build.
- A partial(!) port of Sun's ZFS is also
made available. It's not at the state
where it can be used, but should be a good starting point
for an experienced kernel hacker to continue working.
See Andrew's mail to tech-kern
for further directions.
[Tags: openat, smp, zfs]
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