Hi, > > Hi Rik, > > > > Thanks for reviewing the patches. I wanted to have better understanding of > > where all does it help to associate a bio to the group of process who > > created/owned the page. Hence few thoughts. > > > > When a bio is submitted to IO scheduler, it needs to determine the group > > bio belongs to and group which should be charged to. There seem to be two > > methods. > > > > - Attribute the bio to cgroup submitting process belongs to. > > - For async requests, track the original owner hence cgroup of the page > > and charge that group for the bio. > > > > One can think of pros/cons of both the approaches. > > > > - The primary use case of tracking async context seems be that if a > > process T1 in group G1 mmaps a big file and then another process T2 in > > group G2, asks for memory and triggers reclaim and generates writes of > > the file pages mapped by T1, then these writes should not be charged to > > T2, hence blkio_cgroup pages. > > > > But the flip side of this might be that group G2 is a low weight group > > and probably too busy also right now, which will delay the write out > > and possibly T2 will wait longer for memory to be allocated. In order to avoid this wait, dm-ioband issues IO which has a page with PG_Reclaim as early as possible. > > - At one point of time Andrew mentioned that buffered writes are generally a > > big problem and one needs to map these to owner's group. Though I am not > > very sure what specific problem he was referring to. Can we attribute > > buffered writes to pdflush threads and move all pdflush threads in a > > cgroup to limit system wide write out activity? I think that buffered writes also should be controlled per cgroup as well as synchronous writes. > > - Somebody also gave an example where there is a memory hogging process and > > possibly pushes out some processes to swap. It does not sound fair to > > charge those proccess for that swap writeout. These processes never > > requested swap IO. I think that swap writeouts should be charged to the memory hogging process, because the process consumes more resources and it should get a penalty. > > - If there are multiple buffered writers in the system, then those writers > > can also be forced to writeout some pages to disk before they are > > allowed to dirty more pages. As per the page cache design, any writer > > can pick any inode and start writing out pages. So it can happen a > > weight group task is writting out pages dirtied by a lower weight group > > task. If, async bio is mapped to owner's group, it might happen that > > higher weight group task might be made to sleep on lower weight group > > task because request descriptors are all consumed up. As mentioned above, in dm-ioband, the bio is charged to the page owner and issued immediately. > > It looks like there does not seem to be a clean way which covers all the > > cases without issues. I am just trying to think, what is a simple way > > which covers most of the cases. Can we just stick to using submitting task > > context to determine a bio's group (as cfq does). Which can result in > > following. > > > > - Less code and reduced complexity. > > > > - Buffered writes will be charged to pdflush and its group. If one wish to > > limit buffered write activity for pdflush, one can move all the pdflush > > threads into a group and assign desired weight. Writes submitted in > > process context will continue to be charged to that process irrespective > > of the fact who dirtied that page. > > What if we wanted to control buffered write activity per group? If a > group keeps dirtying pages, we wouldn't want it to dominate the disk > IO capacity at the expense of other cgroups (by dominating the writes > sent down by pdflush). Yes, I think that is true. > > - swap activity will be charged to kswapd and its group. If swap writes > > are coming from process context, it gets charged to process and its > > group. > > > > - If one is worried about the case of one process being charged for write > > out of file mapped by another process during reclaim, then we can > > probably make use of memory controller and mount memory controller and > > io controller together on same hierarchy. I am told that with memory > > controller, group's memory will be reclaimed by the process requesting > > more memory. If that's the case, then IO will automatically be charged > > to right group if we use submitting task context. > > > > I just wanted to bring this point forward for more discussions to know > > what is the right thing to do? Use bio tracking or not. Thanks for bringing it forward. Thanks, Ryo Tsuruta -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
