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Re: [Xen-devel] [PATCH v10 03/19] qspinlock: Add pending bit

On 05/14/2014 03:13 PM, Radim KrÄmÃÅ wrote:
2014-05-14 19:00+0200, Peter Zijlstra:
On Wed, May 14, 2014 at 06:51:24PM +0200, Radim KrÄmÃÅ wrote:
I've seen merit in pvqspinlock even with slightly slower first-waiter,
so I would have happily sacrificed those horrible branches.
(I prefer elegant to optimized code, but I can see why we want to be
  strictly better than ticketlock.)
Peter mentioned that we are focusing on bare-metal patches, so I'll
withold my other paravirt rants until they are polished.
(It was an ambiguous sentence, I have comments for later patches.)

Well, paravirt must happen too, but comes later in this series, patch 3
which we're replying to is still very much in the bare metal part of the
(I think that bare metal spans the first 7 patches.)

I've not had time yet to decode all that Waiman has done to make
paravirt work.

But as a general rule I like patches that start with something simple
and working and then optimize it, this series doesn't seem to quite
grasp that.

And to forcefully bring this thread a little bit on-topic:

Pending-bit is effectively a lock in a lock, so I was wondering why
don't we use more pending bits; advantages are the same, just diminished
by the probability of having an ideally contended lock:
  - waiter won't be blocked on RAM access if critical section (or more)
    ends sooner
  - some unlucky cacheline is not forgotten
  - faster unlock (no need for tail operations)
(- ?)
disadvantages are magnified:
  - increased complexity
  - intense cacheline sharing
    (I thought that this is the main disadvantage of ticketlock.)
(- ?)

One bit still improved performance, is it the best we got?
So, the advantage of one bit is that if we use a whole byte for 1 bit we
can avoid some atomic ops.

The entire reason for this in-word spinner is to amortize the cost of
hitting the external node cacheline.
Every pending CPU removes one length of the critical section from the
delay caused by cacheline propagation and really cold cache is
hundreds(?) of cycles, so we could burn some to ensure correctness and
still be waiting when the first pending CPU unlocks.

Assuming that taking a spinlock is fairly frequent in the kernel, the node structure cacheline won't be so cold after all.

So traditional locks like test-and-test and the ticket lock only ever
access the spinlock word itsef, this MCS style queueing lock has a
second (and, see my other rants in this thread, when done wrong more
than 2) cacheline to touch.

That said, all our benchmarking is pretty much for the cache-hot case,
so I'm not entirely convinced yet that the one pending bit makes up for
it, it does in the cache-hot case.
Yeah, we probably use the faster pre-lock quite a lot.
Cover letter states that queue depth 1-3 is a bit slower than ticket
spinlock, so we might not be losing if we implemented a faster
in-word-lock of this capacity.  (Not that I'm a fan of the hybrid lock.)

I had tried an experimental patch with 2 pending bits. However, the benchmark test that I used show the performance is even worse than without any pending bit. I probably need to revisit that later as to why this is the case. As for now, I will focus on just having one pending bit. If we could find a way to get better performance out of more than 1 pending bit later on, we could always submit another patch to do that.

But... writing cache-cold benchmarks is _hard_ :/
Wouldn't clflush of the second cacheline before trying for the lock give
us a rough estimate?

clflush is a very expensive operation and I doubt that it will be indicative of real life performance at all. BTW, there is no way to write a cache-cold benchmark for that 2nd cacheline as any call to spin_lock will likely to access it if there is enough contention.


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