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SubjectRe: [PATCH 0/5 - V2] locks: avoid thundering-herd wake-ups
On Thu, Aug 09 2018, J. Bruce Fields wrote:

> On Fri, Aug 10, 2018 at 11:50:58AM +1000, NeilBrown wrote:
>> You're good at this game!
>
> Everybody's got to have a hobby, mine is pathological posix locking
> cases....
>
>> So, because a locker with the same "owner" gets a free pass, you can
>> *never* say that any lock which conflicts with A also conflicts with B,
>> as a lock with the same owner as B will never conflict with B, even
>> though it conflicts with A.
>>
>> I think there is still value in having the tree, but when a waiter is
>> attached under a new blocker, we need to walk the whole tree beneath the
>> waiter and detach/wake anything that is not blocked by the new blocker.
>
> If you're walking the whole tree every time then it might as well be a
> flat list, I think?

The advantage of a tree is that it keeps over-lapping locks closer
together.
For it to make a difference you would need a load where lots of threads
were locking several different small ranges, and other threads were
locking large ranges that cover all the small ranges.
I doubt this is common, but it doesn't seem as strange as other things
I've seen in the wild.
The other advantage, of course, is that I've already written the code,
and I like it.

Maybe I'll do a simple-list version, then a patch to convert that to the
clever-tree version, and we can then have something concrete to assess.

Thanks,
NeilBrown
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