On Tue, Nov 1, 2011 at 4:03 PM, Jan Kara <jack@xxxxxxx> wrote: > On Mon 31-10-11 16:14:47, Andy Lutomirski wrote: >> On Mon, Oct 31, 2011 at 4:10 PM, Jan Kara <jack@xxxxxxx> wrote: >> > On Fri 28-10-11 16:37:03, Andy Lutomirski wrote: >> >> On Tue, Oct 25, 2011 at 5:26 AM, Jan Kara <jack@xxxxxxx> wrote: >> >> >> - Why are we calling file_update_time at all? Presumably we also >> >> >> update the time when the page is written back (if not, that sounds >> >> >> like a bug, since the contents may be changed after something saw the >> >> >> mtime update), and, if so, why bother updating it on the first write? >> >> >> Anything that relies on this behavior is, I think, unreliable, because >> >> >> the page could be made writable arbitrarily early by another program >> >> >> that changes nothing. >> >> > We don't update timestamp when the page is written back. I believe this >> >> > is mostly because we don't know whether the data has been changed by a >> >> > write syscall, which already updated the timestamp, or by mmap. That is >> >> > also the reason why we update the timestamp at page fault time. >> >> > >> >> > The reason why file_update_time() blocks for you is probably that it >> >> > needs to get access to buffer where inode is stored on disk and because a >> >> > transaction including this buffer is committing at the moment, your thread >> >> > has to wait until the transaction commit finishes. This is mostly a problem >> >> > specific to how ext4 works so e.g. xfs shouldn't have it. >> >> > >> >> > Generally I believe the attempts to achieve any RT-like latencies when >> >> > writing to a filesystem are rather hopeless. How much hopeless depends on >> >> > the load of the filesystem (e.g., in your case of mostly idle filesystem I >> >> > can imagine some tweaks could reduce your latencies to an acceptable level >> >> > but once the disk gets loaded you'll be screwed). So I'd suggest that >> >> > having RT thread just store log in memory (or write to a pipe) and have >> >> > another non-RT thread write the data to disk would be a much more robust >> >> > design. >> >> >> >> Windows seems to do pretty well at this, and I think it should be fixable on >> >> Linux too. "All" that needs to be done is to remove the pte_wrprotect from >> >> page_mkclean_one. The fallout from that might be unpleasant, though, but >> >> it would probably speed up a number of workloads. >> > Well, but Linux's mm pretty much depends the pte_wrprotect() so that's >> > unlikely to go away in a forseeable future. The reason is that we need to >> > reliably account the number of dirty pages so that we can throttle >> > processes that dirty too much of memory and also protect agaist system >> > going into out-of-memory problems when too many pages would be dirty (and >> > thus hard to reclaim). Thus we create clean pages as write-protected, when >> > they are first written to, we account them as dirtied and unprotect them. >> > When pages are cleaned by writeback, we decrement number of dirty pages >> > accordingly and write-protect them again. >> >> What about skipping pte_wrprotect for mlocked pages and continuing to >> account them dirty even if they're actually clean? This should be a >> straightforward patch except for the effect on stable pages for >> writeback. (It would also have unfortunate side effects on >> ctime/mtime without my other patch to rearrange that code.) > Well, doing proper dirty accounting would be a mess (you'd have to > unaccount dirty pages during munlock etc.) and I'm not sure what all would > break when page writes would not be coupled with page faults. So I don't > think it's really worth it. I'll add it to my back burner. I haven't figured out all (any?) of the accounting yet. > > Avoiding IO during a minor fault would be a decent thing which might be > worth pursuing. As you properly noted "stable pages during writeback" > requirement is one obstacle which won't be that trivial to avoid though... There's an easy solution that would be good enough for me: add a mount option to turn off stable pages. Is the other problem just a race, perhaps? __block_page_mkwrite calls __block_write_begin (which calls get_block, which I think is where the latency comes from) *before* wait_on_page_writeback, which means that there might not be any space allocated yet. --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
