On Tue 01-11-11 18:51:04, Andy Lutomirski wrote: > On Tue, Nov 1, 2011 at 4:10 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > > On Tue, Nov 1, 2011 at 4:03 PM, Jan Kara <jack@xxxxxxx> wrote: > >> 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. > > I think I'm right (other than calling it a race). If I change my code to do: > > - map the file (with MCL_FUTURE set) > - fallocate > - dirty all pages > - fsync > - dirty all pages again > > in the non-real-time thread, then a short test that was a mediocre > reproducer seems to work. > > This is annoying, though -- I'm not generating twice as much write I/O > as I used to. Is there any way to force the delalloc code to do its > thing without triggering writeback? I don't think fallocate has this > effect. fallocate() will preallocate blocks on disk backing the mapped page. That should get rid of latency in __block_write_begin(). Extents will still be marked as uninitialized, but conversion from uninitialized to initialized state happens during writeback / IO completion so you should not care much about it. 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
