On Thu, Nov 03, 2016 at 09:40:12PM +0100, Jan Kara wrote: > On Wed 02-11-16 11:32:04, Kirill A. Shutemov wrote: > > Yes, buffer_head list doesn't scale. That's the main reason (along with 4) > > why syscall-based IO sucks. We spend a lot of time looking for desired > > block. > > > > We need to switch to some other data structure for storing buffer_heads. > > Is there a reason why we have list there in first place? > > Why not just array? > > > > I will look into it, but this sounds like a separate infrastructure change > > project. > > As Christoph said iomap code should help you with that and make things > simpler. If things go as we imagine, we should be able to pretty much avoid > buffer heads. But it will take some time to get there. Just to clarify: is it show-stopper or we can live with buffer_head list for now? > > > 2) PMD-sized pages result in increased space & memory usage. > > > > Space? Do you mean disk space? Not really: we still don't write beyond > > i_size or into holes. > > > > Behaviour wrt to holes may change with mmap()-IO as we have less > > granularity, but the same can be seen just between different > > architectures: 4k vs. 64k base page size. > > Yes, I meant different granularity of mmap based IO. And I agree it isn't a > new problem but the scale of the problem is much larger with 2MB pages than > with say 64K pages. And actually the overhead of higher IO granularity of > 64K pages has been one of the reasons we have switched SLES PPC kernels > from 64K pages to 4K pages (we've got complaints from customers). I guess fadvise()/madvise() hints for opt-in/opt-out should be good enough to deal with this. I probably need to wire them up. > > > 3) In ext4 we have to estimate how much metadata we may need to modify when > > > allocating blocks underlying a page in the worst case (you don't seem to > > > update this estimate in your patch set). With 2048 blocks underlying a page, > > > each possibly in a different block group, it is a lot of metadata forcing > > > us to reserve a large transaction (not sure if you'll be able to even > > > reserve such large transaction with the default journal size), which again > > > makes things slower. > > > > I didn't saw this on profiles. And xfstests looks fine. I probably need to > > run them with 1k blocks once again. > > You wouldn't see this in profiles - it is a correctness thing. And it won't > be triggered unless the file is heavily fragmented which likely does not > happen with any test in xfstests. If it happens you'll notice though - the > filesystem will just report error and shut itself down. Any suggestion how I can simulate this situation? > > The numbers below generated with fio. The working set is relatively small, > > so it fits into page cache and writing set doesn't hit dirty_ratio. > > > > I think the mmap performance should be enough to justify initial inclusion > > of an experimental feature: it useful for workloads that targets mmap()-IO. > > It will take time to get feature mature anyway. > > I agree it will take time for feature to mature so I'me fine with > suboptimal performance in some cases. But I'm not fine with some of the > hacks you do currently because code maintenability is an issue even if > people don't actually use the feature... Hm. Okay, I'll try to check what I can do to make it more maintainable. My worry is that it will make the patchset even bigger... > > Configuration: > > - 2x E5-2697v2, 64G RAM; > > - INTEL SSDSC2CW24; > > - IO request size is 4k; > > - 8 processes, 512MB data set each; > > The numbers indeed look interesting for mmaped case. Can you post the fio > cmdline? I'd like to compare profiles... fio \ --directory=/mnt/ \ --name="$engine-$rw" \ --ioengine="$engine" \ --rw="$rw" \ --size=512M \ --invalidate=1 \ --numjobs=8 \ --runtime=60 \ --time_based \ --group_reporting -- Kirill A. Shutemov -- 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
