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 :)
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