Re: [reiserfs-list] Re: [PATCH] Significant performace improvements on reiserfs systems

[Nikita Danilov]

Andrew Morton writes:
 > 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

In Solaris, before spinning on a busy spin-lock, thread checks whether
spin-lock holder runs on the same processor. If so, thread goes to sleep
and holder wakes it up on spin-lock release. The same, I guess is going
for interrupts that are served as separate threads. This way, one can
re-schedule with spin-locks held.

 > part of the MontaVista patch involved turning spinlocks into
 > semaphores, yes?  That would seem to be the way to go.

Nikita.