Messages in this thread | | | Date | Tue, 29 Jan 2002 19:19:36 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] 2.5: push BKL out of llseek |
| |
Nigel Gamble wrote: > > On 29 Jan 2002, Robert Love wrote: > > On Tue, 2002-01-29 at 20:26, Andrew Morton wrote: > > > Just a little word of caution here. Remember the > > > apache-flock-synchronisation fiasco, where removal > > > of the BKL halved Apache throughput on 8-way x86. > > > > > > This was because the BKL removal turned serialisation > > > on a quick codepath from a spinlock into a schedule(). > > Yes, but the other factor to consider here is why did the extra schedule > take place at all? I think this is a actually a scheduler issue, and > I'm hoping that the new scheduler will behave better in this case. A > call to schedule() should not happen unless the woken process has a > higher priority than the process that did the unlock, but the old > scheduler evidently always calculated this to be the case. But we > really want the process that did the unlock to continue running (until > the end of its timeslice, if not preempted or blocked before then), just > as it would when the lock was a spinlock. It would be interesting to > see whether the new scheduler gets this right. > > Am I remembering the problem correctly? >
I don't think so :)
The problem was that the semaphore was highly contended, so the losing process was explicitly scheduling away.
This doesn't necessarily mean that it was a long-held lock. In this case, it was a short-held lock, but it was also very *frequently* being held and released. This is a scenario where a spinlock is heaps more appropriate than a semaphore.
I don't think we need any locking at all in the default lseek() path, btw. Apart from the non-atomic i_size thing, which is only an issue for 32-bit machines.
- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |