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. 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... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
