Re: vm lock contention reduction

Linus Torvalds wrote:

On Sat, 6 Jul 2002, Andrew Morton wrote:

quoted

Martin is being bitten by the global invalidate more than by the lock.
He increased the size of the kmap pool just to reduce the invalidate
frequency and saw 40% speedups of some stuff.

Those invalidates don't show up nicely on profiles.

I'd like to enhance the profiling support a bit, to create some
infrastructure for doing different kinds of profiles, not just the current
timer-based one (and not just for the kernel).

There's also the P4 native support for "event buffers" or whatever intel
calls them, that allows profiling at a lower level by interrupting not for
every event, but only when the hw buffer overflows.

I haven't had much time to look at the oprofile thing, but what I _have_
seen has made me rather unhappy (especially the horrid system call
tracking kludges).

I'd rather have some generic hooks (a notion of a "profile buffer" and
events that cause us to have to synchronize with it, like process
switches, mmap/munmap - oprofile wants these too), and some generic helper
routines for profiling (turn any eip into a "dentry + offset" pair
together with ways to tag specific dentries as being "worthy" of
profiling).

Depending on the regular timer interrupt will never give good profiles,
simply because it can't be NMI, but also because you then don't have the
choice of using other counters (cache miss etc).

oprofile does much of this, but in a damn ugly manner.

I pinged John about an oprofile merge just the other day actually.
He agrees with you on the syscall table thing.  I think he says
that it could be cleaned up if oprofile was in the tree.  Ditto
the issue with mmap.

I was able to isolate and fix some fairly hairy performance problems
at work with oprofile.  It's a great tool - I use it all the time.  And
it profiles the entire system - right down to file-n-line in some random
shared object.  With NMIs.  It is not just a kernel tool.

So.  John.  Get coding :-)

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