Re: [PATCH] Significant performace improvements on reiserfs systems

Andrew Morton
2001-09-21T01:20:46Z

george anzinger wrote:
>
> ...
> Actually, I rather think that the problem is lock granularity. These
> issues are present in the SMP systems as well. A good solution would be
> one that shortened the spinlock time. No horrid preempt code, just
> tight fast code.
>

This may not be practical.

Take, for example, zap_page_range(). It simply has a lot
of work to do, and it does it inside a spinlock. By doing
it in a tight loop, it's optimal.

There is no way to speed this function up by two or three orders
of magnitude. (Well, there is: don't take the lock at all if
the mm isn't shared, but this is merely an example. There are
other instances).

It seems that for a preemptive kernel to be successful, we need
to globally alter the kernel so that it never holds locks for
more than 500 microseconds. Which is what the conditional_schedule()
(aka cooperative multitasking :)) patches do.

It seems that there are no magic bullets, and low latency will
forever have a global impact on kernel design, unless a way is
found to reschedule with locks held. I recall that a large
part of the MontaVista patch involved turning spinlocks into
semaphores, yes? That would seem to be the way to go.