On Tue, Apr 19, 2011 at 10:30:22AM -0400, Vivek Goyal wrote: [..] > > > > In XFS, you could probably do this at the transaction reservation > > stage where log space is reserved. We know everything about the > > transaction at this point in time, and we throttle here already when > > the journal is full. Adding cgroup transaction limits to this point > > would be the place to do it, but the control parameter for it would > > be very XFS specific (i.e. number of transactions/s). Concurrency is > > not an issue - the XFS transaction subsystem is only limited in > > concurrency by the space available in the journal for reservations > > (hundred to thousands of concurrent transactions). > > Instead of transaction per second, can we implement some kind of upper > limit of pending transactions per cgroup. And that limit does not have > to be user tunable to begin with. The effective transactions/sec rate > will automatically be determined by IO throttling rate of the cgroup > at the end nodes. > > I think effectively what we need is that the notion of parallel > transactions so that transactions of one cgroup can make progress > independent of transactions of other cgroup. So if a process does > an fsync and it is throttled then it should block transaction of > only that cgroup and not other cgroups. > > You mentioned that concurrency is not an issue in XFS and hundreds of > thousands of concurrent trasactions can progress depending on log space > available. If that's the case, I think to begin with we might not have > to do anything at all. Processes can still get blocked but as long as > we have enough log space, this might not be a frequent event. I will > do some testing with XFS and see can I livelock the system with very > low IO limits. Wow, XFS seems to be doing pretty good here. I created a group of 1 bytes/sec limit and wrote few bytes in a file and write quit it (vim). That led to an fsync and process got blocked. From a different cgroup, in the same directory I seem to be able to do all other regular operations like ls, opening a new file, editing it etc. ext4 will lockup immediately. So concurrent transactions do seem to work in XFS. Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
