Hello, On Thu, Mar 28, 2013 at 03:33:43PM -0400, Vivek Goyal wrote: > I am curious why out of order bio is a problem. Doesn't the elevator > already merge bio's with existing requests and if merging does not > happen then requests are sorted in order. So why ordering is not > happening properly with dm-crypt. What additional info dm-crypt has > that it can do better ordering than IO scheduler. Hmmm... well, for one, it doesn't only change ordering. It also changes the timings. Before iosched would get contiguous stream of IOs when the queue gets unplugged (BTW, how does dm crypt handling plugging? If not handled properly, it could definitely affect a lot of things.) With multiple threads doing encryption in the middle, the iosched could get scattered IOs which could easily span multiple millisecs. Even if context tagging was done properly, it could easily lead to much less efficient IO patterns to hardware. Keeping IO order combined with proper plug handling would not only keep the ordering constant but also the relative timing of events, which is an important factor when scheduling IOs. > CFQ might seeing more performance hit because we maintain per > process queues and kernel threads might not be sharing the IO context > (i am not sure). So if all the crypto threads can share the IO > context, atleast it will make sure all IO from them goes into a > single queue. Right, this is important too although I fail to see how workqueue vs. custom dispatch would make any difference here. dm-crypt should definitely be using bio_associate_current(). Thanks. -- tejun -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
