On Fri, Mar 22, 2013 at 2:20 PM, <Valdis.Kletnieks@xxxxxx> wrote: > On Fri, 22 Mar 2013 13:53:45 -0700, Raymond Jennings said: > >> The first heap would be synchronous requests such as reads and syncs >> that someone in userspace is blocking on. >> >> The second is background I/O like writeback and readahead. >> >> The same distinction that CFQ completely makes. > > Again, this may or may not be a win, depending on the exact workload. > > If you are about to block on a userspace read, it may make sense to go ahead > and tack a readahead on the request "for free" - at 100MB/sec transfer and 10ms > seeks, reading 1M costs the same as a seek. If you read 2M ahead and save 3 > seeks later, you're willing. Of course, the *real* problem here is that how > much readahead to actually do needs help from the VFS and filesystem levels - > if there's only 600K more data before the end of the current file extent, doing > more than 600K of read-ahead is a loss. That sounds more like an argument for pushing readahead logic up into the fs and vfs levels. If it were up to me, linear/physical readahead would be disabled entirely. Speaking of which, I hear that tux3 is in the works to take more charge of bossing the block layer around. > Meanwhile, over on the write side of the fence, unless a program is > specifically using O_DIRECT, userspace writes will get dropped into the cache > and become writeback requests later on. So the vast majority of writes will > usually be writebacks rather than syncronous writes. > > So in many cases, it's unclear how much performance CFQ gets from making > the distinction (and I'm positive that given a sufficient supply of pizza and > caffeine, I could cook up a realistic scenario where CFQ's behavior makes > things worse)... > > Did I mention this stuff is tricky? :) > _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
