On Sun, Sep 10, 2017 at 11:41:58PM +0200, Gerd Gerats wrote: > When using futex as a condition variable, for example: to manage a > threadpool, there may be a lot of threads inside the futex_wait to sleep on > this futex. The futex_hash_bucket consists therefore of many struct futex_q > for the same futex. > > On bad luck another futex, used as mutex, hashed into the same bucket. > Every futex_wake on this mutex, has to scan the whole chain of above waiter > to find the struct futex_q for this mutex. For non-unusual threadpool sizes > of more than 20, this should be a considerable effort. > > I therefore suggest to include in the hash-bucketchain only one struct > futex_q per futex and to queue additional waiter in an extrachain at the > 'top' futex_q entry. Thus different futex are isolated from each other, the > cost of a hash collision is reduced.
So I don't dislike that idea.. however > To show the idea, I added a sample patch. Here, the plist is exchanged for > a futex-specific implementation. kernel/pring.h is certainly not not the > right place. So I suppose the purpose of that plist in futex is to enable waking up the highest prio waiter, but with the advent of SCHED_DEADLINE that no longer works. I think Thomas resisted going the RB-tree route earlier..
