On Fri, Aug 16, 2013 at 04:18:33PM -0700, Andy Lutomirski wrote: > On Fri, Aug 16, 2013 at 3:02 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > On Fri, Aug 16, 2013 at 07:37:25AM +1000, Dave Chinner wrote: > >> On Thu, Aug 15, 2013 at 08:17:18AM -0700, Andy Lutomirski wrote: > >> > On Thu, Aug 15, 2013 at 12:11 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> > > On Wed, Aug 14, 2013 at 11:14:37PM -0700, Andy Lutomirski wrote: > >> > >> On Wed, Aug 14, 2013 at 11:01 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> > >> > On Wed, Aug 14, 2013 at 09:32:13PM -0700, Andy Lutomirski wrote: > >> > >> >> On Wed, Aug 14, 2013 at 7:10 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> > >> >> > On Wed, Aug 14, 2013 at 09:11:01PM -0400, Theodore Ts'o wrote: > >> > >> >> >> On Wed, Aug 14, 2013 at 04:38:12PM -0700, Andy Lutomirski wrote: > >> > >> >> >> > > It would be better to write zeros to it, so we aren't measuring the > >> > >> >> >> > > cost of the unwritten->written conversion. > >> > >> >> >> > > >> > >> >> >> > At the risk of beating a dead horse, how hard would it be to defer > >> > >> >> >> > this part until writeback? > >> > >> >> >> > >> > >> >> >> Part of the work has to be done at write time because we need to > >> > >> >> >> update allocation statistics (i.e., so that we don't have ENOSPC > >> > >> >> >> problems). The unwritten->written conversion does happen at writeback > >> > >> >> >> (as does the actual block allocation if we are doing delayed > >> > >> >> >> allocation). > >> > >> >> >> > >> > >> >> >> The point is that if the goal is to measure page fault scalability, we > >> > >> >> >> shouldn't have this other stuff happening as the same time as the page > >> > >> >> >> fault workload. > >> > >> >> > > >> > >> >> > Sure, but the real problem is not the block mapping or allocation > >> > >> >> > path - even if the test is changed to take that out of the picture, > >> > >> >> > we still have timestamp updates being done on every single page > >> > >> >> > fault. ext4, XFS and btrfs all do transactional timestamp updates > >> > >> >> > and have nanosecond granularity, so every page fault is resulting in > >> > >> >> > a transaction to update the timestamp of the file being modified. > >> > >> >> > >> > >> >> I have (unmergeable) patches to fix this: > >> > >> >> > >> > >> >> http://comments.gmane.org/gmane.linux.kernel.mm/92476 > >> > >> > > >> > >> > The big problem with this approach is that not doing the > >> > >> > timestamp update on page faults is going to break the inode change > >> > >> > version counting because for ext4, btrfs and XFS it takes a > >> > >> > transaction to bump that counter. NFS needs to know the moment a > >> > >> > file is changed in memory, not when it is written to disk. Also, NFS > >> > >> > requires the change to the counter to be persistent over server > >> > >> > failures, so it needs to be changed as part of a transaction.... > >> > >> > >> > >> I've been running a kernel that has the file_update_time call > >> > >> commented out for over a year now, and the only problem I've seen is > >> > >> that the timestamp doesn't get updated :) > >> > >> > >> > > >> > [...] > >> > > >> > > If a filesystem is providing an i_version value, then NFS uses it to > >> > > determine whether client side caches are still consistent with the > >> > > server state. If the filesystem does not provide an i_version, then > >> > > NFS falls back to checking c/mtime for changes. If files on the > >> > > server are being modified without either the tiemstamps or i_version > >> > > changing, then it's likely that there will be problems with client > >> > > side cache consistency.... > >> > > >> > I didn't think of that at all. > >> > > >> > If userspace does: > >> > > >> > ptr = mmap(...); > >> > ptr[0] = 1; > >> > sleep(1); > >> > ptr[0] = 2; > >> > sleep(1); > >> > munmap(); > >> > > >> > Then current kernels will mark the inode changed on (only) the ptr[0] > >> > = 1 line. My patches will instead mark the inode changed when munmap > >> > is called (or after ptr[0] = 2 if writepages gets called for any > >> > reason). > >> > > >> > I'm not sure which is better. POSIX actually requires my behavior > >> > (which is most irrelevant). > >> > >> Not by my reading of it. Posix states that c/mtime needs to be > >> updated between the first access and the next msync() call. We > >> update mtime on the first access, and so therefore we conform to the > >> posix requirement.... > >> > >> > My behavior also means that, if an NFS > >> > client reads and caches the file between the two writes, then it will > >> > eventually find out that the data is stale. > >> > >> "eventually" is very different behaviour to the current behaviour. > >> > >> My understanding is that NFS v4 delegations require the underlying > >> filesystem to bump the version count on *any* modification made to > >> the file so that delegations can be recalled appropriately. > > > > Delegations at least shouldn't be an issue here: they're recalled on the > > open. > > Can you translate that into clueless-non-NFS-expert? :) An NFS "delegation" is roughly the same thing as what's called a "lease" by the linux vfs or an "OpLock" in SMB. It's a lock that is recalled from the holder on certain conflicting operations. (Basically a way to tell a client "you're the only one using this file, feel free to cache it until I tell you otherwise".) Delegations are recalled on conflicting opens, so by the time you get to IO there shouldn't be any. I don't think they're really relevant to this discussion. --b. > > Anyway, I'm sending patches in a sec. Dave (Hansen), want to test? I > played with will-it-scale a bit, but I don't really know what I'm > doing. > > --Andy -- 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
