Hey, Vivek. (cc'ing Rakesh and Chad who work on iosched in google).
On Thu, Sep 13, 2012 at 10:53:41AM -0400, Vivek Goyal wrote: > Biggest problem with blkcg CFQ implementation is idling on cgroup. If > we don't idle on cgroup, then we don't get the service differentiaton > for most of the workloads and if we do idle then performance starts > to suck very soon (The moment few cgroups are created). And hierarchy > will just exacertbate this problem because then one will try to idle > at each group in hierarchy. > > This problem is something similar to CFQ's idling on sequential queues > and iopriority. Because we never idled on random IO queue, ioprios never > worked on random IO queues. And same is true for buffered write queues. > Similary, if you don't idle on groups, then for most of the workloads, > service differentiation is not visible. Only the one which are highly > sequential on nature, one can see service differentiation. > > That's one fundamental problem for which we need to have a good answer > before we try to do more work on blkcg. Because we can write as much > code but at the end of the day it might still not be useful because > of the above mentioned issue I faced. I talked with Rakesh about this as the modified cfq-iosched used in google supports proper hierarchy and the feature is heavily depended upon. I was told that nesting doesn't really change anything. The only thing which matters is the number of active cgroups and whether they're nested or how deep doesn't matter - IIUC there's no need to idle for internal nodes if they don't have IOs pending. He draw me some diagrams which made sense for me and the code apparently actually works, so there doesn't seem to be any fundamental issue in implementing hierarchy support in cfq. Thoughts? Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
