On Thu, Dec 22, 2011 at 11:41 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote: > On Thu, Dec 22, 2011 at 12:57:40PM +0000, Stefan Hajnoczi wrote: >> On Wed, Dec 21, 2011 at 1:00 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote: >> If you're stumped by the performance perhaps compare blktraces of the >> request approach vs the bio approach. We're probably performing I/O >> more CPU-efficiently but the I/O pattern itself is worse. > > You mean I/O scheduler have many techniques to do well in I/O pattern? > That's what I want to discuss in this RFC. > > I guess request layer have many techniques proved during long time > to do well I/O but BIO-based drvier ignores them for just reducing locking > overhead. Of course, we can add such techniques to BIO-batch driver like > custom-batch in this series. But it needs lots of work, is really duplication, > and will have a problem on maintenance. > > I would like to listen opinions whether this direction is good or bad. This series is a good platform for performance analysis but not something that should be merged IMO. As you said it duplicates work that I/O schedulers and the request-based block layer do. If other drivers start taking this approach too then the duplication will be proliferated. The value of this series is that you have a prototype to benchmark and understand the bottlenecks in virtio-blk and the block layer better. The results do not should that bypassing the I/O scheduler is always a win. The fact that you added batching suggests there is some benefit to what the request-based code path does. So find out what's good about the request-based code path and how to get the best of both worlds. By the way, drivers for solid-state devices can set QUEUE_FLAG_NONROT to hint that seek time optimizations may be sub-optimal. NBD and other virtual/pseudo device drivers set this flag. Should virtio-blk set it and how does it affect performance? Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
