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SubjectRE: [PATCH 0/4] locks: avoid thundering-herd wake-ups
On Wed, Aug 08 2018, Frank Filz wrote:

>> On Wed, Aug 08, 2018 at 03:54:45PM -0400, J. Bruce Fields wrote:
>> > On Wed, Aug 08, 2018 at 11:51:07AM +1000, NeilBrown wrote:
>> > > If you have a many-core machine, and have many threads all wanting
>> > > to briefly lock a give file (udev is known to do this), you can get
>> > > quite poor performance.
>> > >
>> > > When one thread releases a lock, it wakes up all other threads that
>> > > are waiting (classic thundering-herd) - one will get the lock and
>> > > the others go to sleep.
>> > > When you have few cores, this is not very noticeable: by the time
>> > > the 4th or 5th thread gets enough CPU time to try to claim the lock,
>> > > the earlier threads have claimed it, done what was needed, and
> released.
>> > > With 50+ cores, the contention can easily be measured.
>> > >
>> > > This patchset creates a tree of pending lock request in which
>> > > siblings don't conflict and each lock request does conflict with its
> parent.
>> > > When a lock is released, only requests which don't conflict with
>> > > each other a woken.
>> >
>> > Are you sure you aren't depending on the (incorrect) assumption that
>> > "X blocks Y" is a transitive relation?
>> >
>> > OK I should be able to answer that question myself, my patience for
>> > code-reading is at a real low this afternoon....
>>
>> In other words, is there the possibility of a tree of, say, exclusive
> locks with
>> (offset, length) like:
>>
>> (0, 2) waiting on (1, 2) waiting on (2, 2) waiting on (0, 4)
>>
>> and when waking (0, 4) you could wake up (2, 2) but not (0, 2), leaving a
> process
>> waiting without there being an actual conflict.
>
> That implies that the order the locks were received in was:
>
> (0,4)
> (2,2)
> (1,2)
> (0,2)
>
> But couldn't (0,2) have been made only dependent on (0,4)?

Correct. (0,2) would be a child if (0,4), but a sibling of (2,2).

> Of course then
> (1,2) is dependent on BOTH (2,2) and (0,2). Does this tree logic handle that
> case?

No, there is no support for double dependencies. It is still possible
for a lock request to be woken up which, which still cannot be
satisfied.
When one block lock is unlocked, a pending request might then be queued
against a different blocking lock.

>
> On the other hand, there might be a fairness reason to make (0,2) wait for
> (1,2) even though it could have been granted concurrently with (2,2) since
> this dependency tree also preserves some of the order of lock requests.

The locking API doesn't promise fairness, and I don't think we should
try too hard to achieve it. Certainly we shouldn't actively fight it
(so no LIFO queuing) but if we try harder than that I suspect it would
just be needless complexity.

Thanks,
NeilBrown


>
> Frank
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