Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation

[Waiman Long]

On 07/18/2013 03:42 AM, Ingo Molnar wrote:
> * Waiman Long<waiman.long@xxxxxx>  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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