On 5/28/20 11:02 AM, Sedat Dilek wrote: > On Tue, May 26, 2020 at 10:59 PM Jens Axboe <axboe@xxxxxxxxx> wrote: >> >> We technically support this already through io_uring, but it's >> implemented with a thread backend to support cases where we would >> block. This isn't ideal. >> >> After a few prep patches, the core of this patchset is adding support >> for async callbacks on page unlock. With this primitive, we can simply >> retry the IO operation. With io_uring, this works a lot like poll based >> retry for files that support it. If a page is currently locked and >> needed, -EIOCBQUEUED is returned with a callback armed. The callers >> callback is responsible for restarting the operation. >> >> With this callback primitive, we can add support for >> generic_file_buffered_read(), which is what most file systems end up >> using for buffered reads. XFS/ext4/btrfs/bdev is wired up, but probably >> trivial to add more. >> >> The file flags support for this by setting FMODE_BUF_RASYNC, similar >> to what we do for FMODE_NOWAIT. Open to suggestions here if this is >> the preferred method or not. >> >> In terms of results, I wrote a small test app that randomly reads 4G >> of data in 4K chunks from a file hosted by ext4. The app uses a queue >> depth of 32. If you want to test yourself, you can just use buffered=1 >> with ioengine=io_uring with fio. No application changes are needed to >> use the more optimized buffered async read. >> >> preadv for comparison: >> real 1m13.821s >> user 0m0.558s >> sys 0m11.125s >> CPU ~13% >> >> Mainline: >> real 0m12.054s >> user 0m0.111s >> sys 0m5.659s >> CPU ~32% + ~50% == ~82% >> >> This patchset: >> real 0m9.283s >> user 0m0.147s >> sys 0m4.619s >> CPU ~52% >> >> The CPU numbers are just a rough estimate. For the mainline io_uring >> run, this includes the app itself and all the threads doing IO on its >> behalf (32% for the app, ~1.6% per worker and 32 of them). Context >> switch rate is much smaller with the patchset, since we only have the >> one task performing IO. >> >> Also ran a simple fio based test case, varying the queue depth from 1 >> to 16, doubling every time: >> >> [buf-test] >> filename=/data/file >> direct=0 >> ioengine=io_uring >> norandommap >> rw=randread >> bs=4k >> iodepth=${QD} >> randseed=89 >> runtime=10s >> >> QD/Test Patchset IOPS Mainline IOPS >> 1 9046 8294 >> 2 19.8k 18.9k >> 4 39.2k 28.5k >> 8 64.4k 31.4k >> 16 65.7k 37.8k >> >> Outside of my usual environment, so this is just running on a virtualized >> NVMe device in qemu, using ext4 as the file system. NVMe isn't very >> efficient virtualized, so we run out of steam at ~65K which is why we >> flatline on the patched side (nvme_submit_cmd() eats ~75% of the test app >> CPU). Before that happens, it's a linear increase. Not shown is context >> switch rate, which is massively lower with the new code. The old thread >> offload adds a blocking thread per pending IO, so context rate quickly >> goes through the roof. >> >> The goal here is efficiency. Async thread offload adds latency, and >> it also adds noticable overhead on items such as adding pages to the >> page cache. By allowing proper async buffered read support, we don't >> have X threads hammering on the same inode page cache, we have just >> the single app actually doing IO. >> >> Been beating on this and it's solid for me, and I'm now pretty happy >> with how it all turned out. Not aware of any missing bits/pieces or >> code cleanups that need doing. >> >> Series can also be found here: >> >> https://git.kernel.dk/cgit/linux-block/log/?h=async-buffered.5 >> >> or pull from: >> >> git://git.kernel.dk/linux-block async-buffered.5 >> > > Hi Jens, > > I have pulled linux-block.git#async-buffered.5 on top of Linux v5.7-rc7. > > From first feelings: > The booting into the system (until sddm display-login-manager) took a > bit longer. > The same after login and booting into KDE/Plasma. There is no difference for "regular" use cases, only io_uring with buffered reads will behave differently. So I don't think you have longer boot times due to this. > I am building/linking with LLVM/Clang/LLD v10.0.1-rc1 on Debian/testing AMD64. > > Here I have an internal HDD (SATA) and my Debian-system is on an > external HDD connected via USB-3.0. > Primarily, I use Ext4-FS. > > As said above is the "emotional" side, but I need some technical instructions. > > How can I see Async Buffer Reads is active on a Ext4-FS-formatted partition? You can't see that. It'll always be available on ext4 with this series, and you can watch io_uring instances to see if anyone is using it. > Do I need a special boot-parameter (GRUB line)? > > Do I need to activate some cool variables via sysfs? > > Do I need to pass an option via fstab entry? No to all of these, you don't need anything to activate it. You need the program to use io_uring to do buffered reads. > Are any Async Buffer Reads related linux-kconfig options not set? > Which make sense? No kconfig options are needed. -- Jens Axboe