On Tue, Apr 12, 2011 at 11:31:17PM +1000, Dave Chinner wrote: > I don't think so. e.g. in the XFS allocation path we do btree block > readahead, then go do the real work. The real work can end up with a > deeper stack before blocking on locks or completions unrelated to > the readahead, leading to schedule() being called and an unplug > being issued at that point. You might think it contrived, but if > you can't provide a guarantee that it can't happen then I have to > assume it will happen. In addition to the stack issue, which is a killer to this also has latency implications. Before we could submit a synchronous metadata read request inside readpage or writepage and kick it off to the disk immediately, while now it won't get submitted until we block the next time, i.e. have done some more work that could have been used for doing I/O in the background. With the kblockd offload not only have we spent more time but at the point where we finally kick it we also need another context switch. It seem like we really need to go through the filesystems and explicitly flush the plugging queue for such cases. In fact a bio flag marking things as synchronous metadata reads would help, but then again we need to clean up our existing bio flags first.. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
