2013/3/25 Raymond Jennings <shentino@xxxxxxxxx>: > 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? 1. Add your disk scheduler to the kernel sources (Kconfig, Makefile and in block/bfq-iosched.c) 2. Add the bfq scheduler in the kernel config (as a moudle might make sense) 3. Recompile and install your new kernel 4. You can load/unload the module dynamically. Via sysfs you can associate the bfq scheduler with one disk. Happy hacking :) -- --- motzblog.wordpress.com _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
