Il giorno 22/apr/2016, alle ore 20:13, Tejun Heo <tj@xxxxxxxxxx> ha scritto: > Hello, Paolo. > > On Wed, Apr 20, 2016 at 11:32:23AM +0200, Paolo wrote: >> This malfunction seems related to a blkcg behavior that I did not >> expect: the sequential writer changes group continuously. It moves >> from the root group to its correct group, and back. Here is the >> output of >> >> egrep 'insert_request|changed cgroup' trace >> >> over a trace taken with the original version of cfq (seq_write is of >> course the group of the writer): > ... >> For reasons that I don't yet know, group changes are much more >> frequent with bfq, which ultimately causes bfq to fail to isolate the >> writer from the reader. >> >> While I go on trying to understand why, could you please tell me >> whether this fluctuation is normal, and/or point me to documentation from >> which I can better understand this behavior, without bothering you >> further? > > So, a kworker would jump through different workqueues and issue IOs > for different writeback domains and the context can't be tied to the > issuing task. The cgroup membership should be determined directly > from the bio. Yes. My doubt arises from the fact that the only source of intense I/O is the dd (I have executed it alone). In contrast, group changes occur at a high frequency during all the execution of the dd. Apparently I cannot see any other I/O induced by the dd. Journaling issues sync requests. > cfq uses per-cgroup async queue. I'm not sure how this > would map to bfq tho. > It’s the same. But this is the part I’m checking. Thanks, Paolo > Thanks. > > -- > tejun -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
