On 03/08/2012 08:43 AM, Sage Weil wrote: > On Thu, 8 Mar 2012, Ted Ts'o wrote: >> On Wed, Mar 07, 2012 at 10:27:43PM -0800, Sage Weil wrote: >>> >>> This avoids the problem for devices that don't need stable pages, but >>> doesn't help for those that do (btrfs, raid, iscsi, dif/dix, etc.). It >>> seems to me like a more elegant solution would be to COW the page in the >>> address_space so that you get stable writeback pages without blocking. >>> That's clearly more complex, and I'm sure there are a range of issues >>> involved in making that work, but I would hope that it would be doable >>> with generic MM infrastructure so that everyone would benefit. >> >> Well, even doing a COW (or anything that involves messing with page >> tables) is not free. So even if we can make the cost of stable >> writeback pages cheaper, if we can completely avoid the cost, this >> would be good. I'd also rather fix the performance regression sooner >> rather than later, and I suspect the COW solution is not something >> that could be prepared in time for the upcoming merge window. > > Definitely. This patch looks like a fine approach for your situation. I > just don't want the subject to come up without talking about a general > solution. And it's very interesting to hear about a (simple) workload > that is affected by the wait_on_page_writeback(). I'll add a simple workload. I have a soft real-time program that has two threads. One of them fallocates some files, mmaps them, mlocks them, and touches all the pages to prefault them. (This thread has no real-time constraints -- it just needs to keep up.) The other thread writes to the files. On Windows, this works very well. On Linux without stable pages, it almost works. With stable pages, it's a complete disaster. No amount of minimizing the amount of time that pages under writeback can cause writers to sleep will help -- writers *must not wait for io* when writing mlocked, prefaulted pages for my code to work. (The other issue involves file_update_time. I'll send a fix eventually.) FWIW, it would be really nice if there was a way to lock a mapping so hard that accesses are guaranteed to not even cause soft faults. We're far from being able to do that now, though. --Andy -- 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
