On 03/08/2012 10:09 AM, Chris Mason wrote: > > But, why are we writeback for a second or more? Aren't there other > parts of this we would want to fix as well? > > I'm not against only turning on stable pages when they are needed, but > the code that isn't the default tends to be somewhat less used. So it > does increase testing burden when we do want stable pages, and it tends > to make for awkward bugs that are hard to reproduce because someone > neglects to mention it. > > IMHO it's much more important to nail down the 2 second writeback > latency. That's not good. > I think I understand this one. It's do to the sync nature introduced by page_waiting in mkwrite. The system is loaded everything is somewhat 2 second or more in a lag. The 2 sec (or more) comes from the max-dirty-limit/disk-speed so any IO you'll submit will probably be on stable disk 2 sec later. (In theory, any power fail will loose all dirty pages which is in our case max-dirty-limit) Now usually that's fine because everything is queued and waits a bit evenly distributed and you wait, theoretically, only the rate of your IO. But here, all of a sudden, you are not allowed to be queued and you are waiting for the head of queue to be actually done, and the app is just frozen. Actually now when I think of it the pages were already submitted for them to be waited on. So the 2-sec is the depth of the block+scsi+target queues. I guess they can be pretty deep. I have a theory of how we can fix that 2-sec wait, by avoiding writeback of the last n pages of an inode who's mtime is less then 2-sec. This would solve any sequential writer wait penalty, which is Ted's case Thanks Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
