rwlocks: poor performance with adaptive spinning

Jeff Roberson jroberson at
Wed Sep 26 08:52:12 PDT 2007

On Wed, 26 Sep 2007, Attilio Rao wrote:

> 2007/9/26, Jeff Roberson <jroberson at>:
>> On Tue, 25 Sep 2007, John Baldwin wrote:
>>> So I looked at this some more and now I don't understand why the trywait() and
>>> cancel() changes were done.  Some background:
>> I wanted the turnstile chain lock to protect only the association between
>> the turnstile and lock.  This way it further reduces the contention domain
>> for the hard lock/unlock cases.  We sometimes see a lot of contention on
>> the turnstile chain that may be due to hash collisions.  If this reduced
>> contention doesn't help I think it would be possible to revert this
>> change.
> Well, the change jhb is suggesting won't reduce tc_lock contention for
> sure, but it will prevent the turnstile lookup/locking if not
> necessary.
> Mainly, we need a spinlock held in order to protect mtx_lock bitfield
> (as well as rw_lock) by setting waiters flags.
> While in the past code this lock was only tc_lock, currently mtx_lock
> is both protected by tc_lock and ts_lock, which it doesn't seem
> strictly necessary.
> if, for example, you lock tc_lock than you scan the chain and lock
> ts_lock (basically what turnstile_trywait() does) and later you find
> lock is released or that the state of lock changed and you cannot set
> a waiters flag, you did the ts_locking and hash table scanning even if
> it wasn't needed.

Yes I understand, the question is whether the decreased complexity is a 
bigger win than the reduced contention.  The contention is only reduced if 
there are collisions in the table.  So probably jhb is right, we should 
revert this part of the change.  I need to make sure there was not some 
other reason for it.  I had convinced myself at the time that it was 
necessary, however, I can't recall why.

>> I'm not sure why you're calling this O(n).  It's O(n) with the number of
>> hash collisions, but it's a hash lookup.  We should consider using a real
>> hash algorithm here if the distribution is bad.
> Well, the hash lookup (worst case) has O(n) complexity.
> This linear complexity cames from the O(1) * O(n) which is the access
> to the list and the scanning of this one. So this is reduced to the
> same complexity of a linked list.

sure, I just don't think it's common to call a hash O(n).

> However, some time ago I saw empirically that starting from a 'struct
> thread' allocated address the highest level of entropy in the word
> came after a right shift of 10 bits (for ia32) and 12 bits for amd64.
> Currently, turnstile and sleepqueues only uses 8 bits of right
> shifting.

I haven't studied the hash distribution in turnstiles, I know there is 
some debugging code in there for it so jhb must have.  Perhaps he can say 
how it does.

> Thanks,
> Attilio
> -- 
> Peace can only be achieved by understanding - A. Einstein

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