On Sun, Oct 20, 2013 at 08:11:12PM +0800, Hong Zhiguo wrote: > From: Hong Zhiguo <zhiguohong@xxxxxxxxxxx> > > Why > ==== > Pointed out by Vivek: Tokens generated during idle period should > be trimmed. Otherwise a huge bio may be permited immediately. > Overlimit behaviour may be observed during short I/O throughput > test. > > Vivek also pointed out: We should not over-trim for hierarchical > groups. Suppose a subtree of groups are idle when a big bio comes. > The token of the child group is trimmed and not enough. So the bio is > queued on the child group. After some jiffies the child group reserved > enough tokens and the bio climbs up. If we trim the parent group at > this time again, this bio will wait too much time than expected. > > Analysis > ======== > When the bio is queued on child group, all it's ancestor groups > becomes non-idle. They should start to reserve tokens for that > bio from this moment. And their reserved tokens before should be > trimmed at this moment. > > How > ==== > service_queue now has a new member nr_queued_tree[2], to represent > the the number of bios waiting on the subtree rooted by this sq. > > When a bio is queued on the hierarchy first time, nr_queued_tree > of all ancestors and the child group itself are increased. When a > bio climbs up, nr_queued_tree of the child group is decreased. > > When nr_queued_tree turns from zero to one, the tokens reserved > before are trimmed. And after this switch, this group will never > be trimmed to reserve tokens for the bio waiting on it's descendant > group. > Hi Hong, This approach looks good in general. Only downside I can think of updation of nr_requests throughout the hierarchy. So deeper the hierarchy, higher the overhead. I am not sure if that's a concern or not. I will have a closer look a the patches tomorrow and do some testing too. Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
