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. - 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
