On Tue, 19 Aug 2008 14:43:27 -0400 Ric Wheeler <rwheeler@xxxxxxxxxx> wrote: > Andrew Morton wrote: > > On Tue, 19 Aug 2008 07:01:11 -0400 Ric Wheeler <rwheeler@xxxxxxxxxx> wrote: > > > > > >> It would be great to be able to use this batching technique for faster > >> devices, but we currently sleep 3-4 times longer waiting to batch for an > >> array than it takes to complete the transaction. > >> > > > > Obviously, tuning that delay down to the minimum necessary is a good > > thing. But doing it based on commit-time seems indirect at best. What > > happens on a slower disk when commit times are in the tens of > > milliseconds? When someone runs a concurrent `dd if=/dev/zero of=foo' > > when commit times go up to seconds? > > > > Perhaps a better scheme would be to tune it based on how many other > > processes are joining that transaction. If it's "zero" then decrease > > the timeout. But one would need to work out how to increase it, which > > perhaps could be done by detecting the case where process A runs an > > fsync when a commit is currently in progress, and that commit was > > caused by process B's fsync. > > > > But before doing all that I would recommend/ask that the following be > > investigated: > > > > - How effective is the present code? > > > > - What happens when it is simply removed? > > > > - Add instrumentation (a counter and a printk) to work out how > > many other tasks are joining this task's transaction. > > > > - If the answer is "zero" or "small", work out why. > > > > - See if we can increase its effectiveness. > > > > Because it could be that the code broke. There might be issues with > > higher-level locks which are preventing the batching. For example, if > > all the files which the test app is syncing are in the same directory, > > perhaps all the tasks are piling up on that directory's i_mutex? > > > > One other way to think about this is as a fairly normal queuing problem: > > (1) arrival rate is the rate at which we see new tasks coming into > the code > (2) service time is basically the time spent committing the > transaction to storage > > and we have the assumption that some number of tasks can join a > transaction more or less for "free." > > What the existing code assumes is that all devices have an equal service > time. That worked well as long as we only looked at devices that were > roughly equal (10-20 ms latencies) or used a higher HZ for the kernel > (1000HZ and you don't see this as much as with 100HZ). > > The two key issues that Josef's code tried to address are that first > assumption that all devices have a similar service time and the tie > between how long we wait and the HZ. yes, I see the (indirect) logic. But I wonder about whether there's a direct way of measuring the thing we really want to measure. Also, I'd be heaps less scared of the change if it did commit_time = min(commit_time, one jiffy) > It would seem to be generically > useful to be able to sleep for less than 1 jiffie, not just for file > systems, but maybe also in some other contexts? Sure. -- 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
