On Sat, Mar 23, 2013 at 9:42 AM, Matthias Brugger <matthias.bgg@xxxxxxxxx> wrote: > On 03/23/2013 01:05 AM, Raymond Jennings wrote: > > 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. > > 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? :) > > Oh I'm well aware that it's tricky. but as I said i'm more interested > in learning the api than tuning performance. > > Having a super efficient toaster won't do much good if I can't plug > the darn thing in. > > > If you want to understand the interface, I would recommend to start having a > look to the noop scheduler. It's by far the simplest implementation of a > scheduler. > > For me a good starting point were this slides: > http://www.cs.ccu.edu.tw/~lhr89/linux-kernel/Linux%20IO%20Schedulers.pdf > > Hope that helps you to bring the theory into practice :) > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies@xxxxxxxxxxxxxxxxx > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > Just what I was looking for. Now, how do I enable/disable my scheduler during kernel config? _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
