On Mon, 10 Nov 2008, Peter Zijlstra wrote: > On Mon, 2008-11-10 at 09:19 -0500, Mikulas Patocka wrote: > > On Mon, 10 Nov 2008, Christoph Hellwig wrote: > > > > > On Mon, Nov 10, 2008 at 08:11:51AM -0500, Mikulas Patocka wrote: > > > > For upstream Linux developers: you are holding a spinlock and calling > > > > bdi*_congested functions that can take indefinite amount of time (there > > > > are even users reporting having 50 disks in one logical volume or so). I > > > > think it would be good to move these calls out of spinlocks. > > > > > > Umm, they shouldn't block that long, as that completely defeats their > > > purpose. These functions are mostly used to avoid throwing more I/O at > > > a congested device if pdflush could do more useful things instead. But > > > if it blocks in those functions anyway we wouldn't have to bother using > > > them. Do you have more details about the uses cases when this happens > > > and where the routines spend so much time? > > > > For device mapper, congested_fn asks every device in the tree and make OR > > of their bits --- so if the user has 50 devices, it asks them all. > > > > For md-linear, md-raid0, md-raid1, md-raid10 and md-multipath it does the > > same --- asking every device. > > > > If you have a better idea how to implement congested_fn, say it. > > Fix the infrastructure by adding a function call so that you can have > the individual devices report their congestion state to the aggregate. > > Then congestion_fn can return a valid state in O(1) because the state is > keps up-to-date by the individual state changes. > > IOW, add a set_congested_fn() and clear_congested_fn(). If you have a physical disk that has many LVM volumes on it, you end up in a situation when disk congestion state change is reported to all the volumes. So it will create O(n) problem at the other side. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
