On Tue, Jun 28, 2011 at 10:03:54AM +0800, Shaohua Li wrote: [..] > > > > Any feedback on how to solve this issue is appreciated. > > > Hi Vivek, > > > can we introduce a group think time check in cfq? say in a group the > > > last queue is backed for the group and the queue is a non-idle queue, if > > > the group think time is big, we don't allow the group idle and preempt > > > could happen. The fsync thread is a non-idle queue with Corrado's patch, > > > this allows fast group switch. > > > > In this case regular queue idle is hitting and not group idle. So some > > kind of think time stats probably might be useful for group idle check > > but not necessarily for queue idle. > I thought your problem is group idle issue. fsync uses WRITE_SYNC, which > will make the queue be sync-non-idle because REQ_NOIDLE is set. This is > exactly what Corrado's patch for. a fsync queue itself isn't idle unless > it's the last queue in a group. Am I missing anything? We idle on last queue on sync-noidle tree. So we idle on fysnc queue as it is last queue on sync-noidle tree. That's how we provide protection to all sync-noidle queues against sync-idle queues. Instead of idling on individual quues we do idling in group and that is on service tree. > > > Secondly, for this case think time will change. If you stop idling on > > fsync and jbd threads, both will be dispatching IOs fast and both will > > have small thinktime. We will think that thinktime is small so we > > will enable idle. Then there think time will increase as both will > > get blocked behind each other. And then we will removing idling. So > > looks like we will be oscillating between enabling and disabling > > think time. > That is possible, the think time check (even for queues) always has such > issue. Not sure how severe the issue is. Assume jbd will dispatch > several requests and this will make fsync thread think time big. > > > If we don't allow idling on sync-no-idle queues, then basic CFQ will > > be broken. > Hmm, CFQ only allows idling on sync queues, sync-no-idle queue isn't > allowed idling. See above. Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
