Messages in this thread | | | Date | Thu, 18 Jul 2013 09:40:10 -0400 | From | Waiman Long <> | Subject | Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation |
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On 07/18/2013 03:42 AM, Ingo Molnar wrote: > * Waiman Long<waiman.long@hp.com> wrote: > >>>> + * stealing the lock if come at the right moment, the granting of the >>>> + * lock is mostly in FIFO order. >>>> + * 2. It is faster in high contention situation. >>> Again, why is it faster? >> The current rwlock implementation suffers from a thundering herd >> problem. When many readers are waiting for the lock hold by a writer, >> they will all jump in more or less at the same time when the writer >> releases the lock. That is not the case with qrwlock. It has been shown >> in many cases that avoiding this thundering herd problem can lead to >> better performance. > Btw., it's possible to further optimize this "writer releases the lock to > multiple readers spinning" thundering herd scenario in the classic > read_lock() case, without changing the queueing model. > > Right now read_lock() fast path is a single atomic instruction. When a > writer releases the lock then it makes it available to all readers and > each reader will execute a LOCK DEC instruction which will succeed. > > This is the relevant code in arch/x86/lib/rwlock.S [edited for > readability]: > > __read_lock_failed(): > > 0: LOCK_PREFIX > READ_LOCK_SIZE(inc) (%__lock_ptr) > > 1: rep; nop > READ_LOCK_SIZE(cmp) $1, (%__lock_ptr) > js 1b > > LOCK_PREFIX READ_LOCK_SIZE(dec) (%__lock_ptr) > js 0b > > ret > > This is where we could optimize: instead of signalling to each reader that > it's fine to decrease the count and letting dozens of readers do that on > the same cache-line, which ping-pongs around the numa cross-connect > touching every other CPU as they execute the LOCK DEC instruction, we > could let the _writer_ modify the count on unlock in essence, to the exact > value that readers expect. > > Since read_lock() can never abort this should be relatively > straightforward: the INC above could be left out, and the writer side > needs to detect that there are no other writers waiting and can set the > count to 'reader locked' value - which the readers will detect without > modifying the cache line: > > __read_lock_failed(): > > 0: rep; nop > READ_LOCK_SIZE(cmp) $1, (%__lock_ptr) > js 0b > > ret > > (Unless I'm missing something that is.) > > That way the current write_unlock() followed by a 'thundering herd' of > __read_lock_failed() atomic accesses is transformed into an efficient > read-only broadcast of information with only a single update to the > cacheline: the writer-updated cacheline propagates in parallel to every > CPU and is cached there. > > On typical hardware this will be broadcast to all CPUs as part of regular > MESI invalidation bus traffic. > > reader unlock will still have to modify the cacheline, so rwlocks will > still have a fundamental scalability limit even in the read-only usecase.
I think that will work. The only drawback that I can see is the fairness argument. The current read/write lock implementation is unfair to the writer. That change will make it even more unfair to the writer and there is no easy way to detect a waiting writer unless we change the structure to add such a field. As a result, a steady stream of readers will have a higher chance of blocking out a writer indefinitely.
Regards, Longman
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