On Thu, Mar 08, 2012 at 12:20:26PM -0800, Boaz Harrosh wrote: > > 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 That won't work in general, *unless* 2 seconds is enough time that the appending writer is done writing to that particular 4k page and moved on to the next 4k block, so nothing touches that page and potentially blocks for however long it takes for the queues to drain. Let's take another example, suppose you have a file-backed mmap region, and you modify the page, and now let's suppose the process is under enough memory pressure that the page cleaner decides to initiate writeback of the page. Now suppose you get unlucky (this is the 1% or 0.1% case; remember, 99th or 99.9 percentile latencies matter), and you try to modify the page in question again. ***THUNK*** your process takes a page fault, and is frozen solid in amber for potentially seconds until the I/O queues drain. Hmm.... let's turn this around. If the issue is checksum calculation, how about trying to solve this problem in some cases by deferring the checksum calculation until right before the block I/O layer is going to schedule the write (i.e., have the I/O submitter provide a callback function which calculates the checksum, which gets called by the BIO layer at the very last moment)? This won't work in all cases (I can see this getting really messy in the software RAID-5/6 case if you don't want to memory copies) but it might solve the problem in at least some of the cases where people care about this. - Ted -- 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
