On Tue 21-05-19 08:23:05, Paolo Valente wrote: > > Il giorno 21 mag 2019, alle ore 00:45, Srivatsa S. Bhat <srivatsa@xxxxxxxxxxxxx> ha scritto: > > > > On 5/20/19 3:19 AM, Paolo Valente wrote: > >> > >> > >>> Il giorno 18 mag 2019, alle ore 22:50, Srivatsa S. Bhat <srivatsa@xxxxxxxxxxxxx> ha scritto: > >>> > >>> On 5/18/19 11:39 AM, Paolo Valente wrote: > >>>> I've addressed these issues in my last batch of improvements for BFQ, > >>>> which landed in the upcoming 5.2. If you give it a try, and still see > >>>> the problem, then I'll be glad to reproduce it, and hopefully fix it > >>>> for you. > >>>> > >>> > >>> Hi Paolo, > >>> > >>> Thank you for looking into this! > >>> > >>> I just tried current mainline at commit 72cf0b07, but unfortunately > >>> didn't see any improvement: > >>> > >>> dd if=/dev/zero of=/root/test.img bs=512 count=10000 oflag=dsync > >>> > >>> With mq-deadline, I get: > >>> > >>> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 3.90981 s, 1.3 MB/s > >>> > >>> With bfq, I get: > >>> 5120000 bytes (5.1 MB, 4.9 MiB) copied, 84.8216 s, 60.4 kB/s > >>> > >> > >> Hi Srivatsa, > >> thanks for reproducing this on mainline. I seem to have reproduced a > >> bonsai-tree version of this issue. Before digging into the block > >> trace, I'd like to ask you for some feedback. > >> > >> First, in my test, the total throughput of the disk happens to be > >> about 20 times as high as that enjoyed by dd, regardless of the I/O > >> scheduler. I guess this massive overhead is normal with dsync, but > >> I'd like know whether it is about the same on your side. This will > >> help me understand whether I'll actually be analyzing about the same > >> problem as yours. > >> > > > > Do you mean to say the throughput obtained by dd'ing directly to the > > block device (bypassing the filesystem)? > > No no, I mean simply what follows. > > 1) in one terminal: > [root@localhost tmp]# dd if=/dev/zero of=/root/test.img bs=512 count=10000 oflag=dsync > 10000+0 record dentro > 10000+0 record fuori > 5120000 bytes (5,1 MB, 4,9 MiB) copied, 14,6892 s, 349 kB/s > > 2) In a second terminal, while the dd is in progress in the first > terminal: > $ iostat -tmd /dev/sda 3 > Linux 5.1.0+ (localhost.localdomain) 20/05/2019 _x86_64_ (2 CPU) > > ... > 20/05/2019 11:40:17 > Device tps MB_read/s MB_wrtn/s MB_read MB_wrtn > sda 2288,00 0,00 9,77 0 29 > > 20/05/2019 11:40:20 > Device tps MB_read/s MB_wrtn/s MB_read MB_wrtn > sda 2325,33 0,00 9,93 0 29 > > 20/05/2019 11:40:23 > Device tps MB_read/s MB_wrtn/s MB_read MB_wrtn > sda 2351,33 0,00 10,05 0 30 > ... > > As you can see, the overall throughput (~10 MB/s) is more than 20 > times as high as the dd throughput (~350 KB/s). But the dd is the > only source of I/O. Yes and that's expected. It just shows how inefficient small synchronous IO is. Look, dd(1) writes 512-bytes. From FS point of view we have to write: full fs block with data (+4KB), inode to journal (+4KB), journal descriptor block (+4KB), journal superblock (+4KB), transaction commit block (+4KB) - so that's 20KB just from top of my head to write 512 bytes... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
