On Mon, Aug 31, 2009 at 11:56 AM, Vivek Goyal<vgoyal@xxxxxxxxxx> wrote: > On Mon, Aug 31, 2009 at 01:34:54PM -0400, Rik van Riel wrote: >> Vivek Goyal wrote: >>> o blkio_cgroup patches from Ryo to track async bios. >>> >>> o This functionality is used to determine the group of async IO from page >>> instead of context of submitting task. >>> >>> Signed-off-by: Hirokazu Takahashi <taka@xxxxxxxxxxxxx> >>> Signed-off-by: Ryo Tsuruta <ryov@xxxxxxxxxxxxx> >>> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx> >> >> This seems to be the most complex part of the code so far, >> but I see why this code is necessary. >> > > 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. > > - 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? > > - 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. > > - 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. > > 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). > > - 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. > > Ryo, any thoughts on this? > > Thanks > Vivek > >> Acked-by: Rik van Riel <riel@xxxxxxxxxx> > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
