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? :) 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-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
