On Tue, Sep 22, 2009 at 07:45:37PM +0800, Jan Kara wrote: > On Tue 22-09-09 07:30:55, Chris Mason wrote: > > > Yes a more general solution would help. I'd like to propose one which > > > works in the other way round. In brief, > > > (1) the VFS give a large enough per-file writeback quota to btrfs; > > > (2) btrfs tells VFS "here is a (seek) boundary, stop voluntarily", > > > before exhausting the quota and be force stopped. > > > > > > There will be two limits (the second one is new): > > > > > > - total nr to write in one wb_writeback invocation > > > - _max_ nr to write per file (before switching to sync the next inode) > > > > > > The per-invocation limit is useful for balance_dirty_pages(). > > > The per-file number can be accumulated across successive wb_writeback > > > invocations and thus can be much larger (eg. 128MB) than the legacy > > > per-invocation number. > > > > > > The file system will only see the per-file numbers. The "max" means > > > if btrfs find the current page to be the last page in the extent, > > > it could indicate this fact to VFS by setting wbc->would_seek=1. The > > > VFS will then switch to write the next inode. > > > > > > The benefit of early voluntarily yield is, it reduced the possibility > > > to be force stopped half way in an extent. When next time VFS returns > > > to sync this inode, it will again be honored the full 128MB quota, > > > which should be enough to cover a big fresh extent. > > > > This is interesting, but it gets into a problem with defining what a > > seek is. On some hardware they are very fast and don't hurt at all. It The hardware capability could be reported in the bdi? > > might be more interesting to make timeslices. > With simple timeslices there's a problem that the time it takes to submit > an IO isn't really related to the time it takes to complete the IO. During > submission we are limited just by availablity of free requests and sizes of > request queues (which might be filled by another thread or by us writing > different inode). Right. When queue is congested, the submission time will be correlated with (someone else's) completion time. So it is still necessary to have a quota of submission time to prevent one single inode takes too much sync (submission) time. > But as I described in my other email, we could probably estimate time it > takes to complete the IO. At least CFQ keeps statistics needed for that. If > we somehow generalized them and put them into BDI, we could probably use > them during writeback... As for randomness, I think write_cache_pages() could get a good estimation by counting the number of page segments it put to io for a single inode, without going for the block layer. Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
