[lustre-discuss] Hidden QoS in Lustre ?
Tung-Han Hsieh
thhsieh at twcp1.phys.ntu.edu.tw
Thu Oct 29 22:37:01 PDT 2020
Dear Simon,
Thank you very much for your useful information. Now we are arranging
the system maintenance date in order to upgrade to Lustre-2.12.5. Then
we will follow your suggestion to see whether this problem could be
fixed.
Here I report a test of under continuous I/O, how the cur_grant_bytes
changed overtime. Again the client runs the following script for
continuous reading in the background:
# The Lustre file system was mounted under /home
while [ 1 ]; do
tar cf - /home/large/data | ssh remote_host "cat > /dev/null"
done
And every 20 mins, in the same client we copied a 600MB file from one
directory to another within Lustre, and check the "cur_grant_bytes" by
the following command running in the same client:
/opt/lustre/sbin/lctl get_param osc.*.cur_grant_bytes
The result is (every line separated by around 20 mins):
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=1880752127
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=1410564096
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=1059201024
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=794400768
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=595800576
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=446850432
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=335137824
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=251353368
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=188515026
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=141386270
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=106039703
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=79529778
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=59647334
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=44735501
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=33551626
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=25163720
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=18872790
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=14154593
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=10615945
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=7961959
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=5971470
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=4478603
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=3358953
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=2519215
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=1889412
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=1417059
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=1062795
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=797097
osc.chome-OST0000-osc-ffff88a039150000.cur_grant_bytes=797097
....
The value 797097 seems to be the minimum. When it dropped to 1062795,
the time of cp dramatically increased from around 1 sec to 1 min. In
addition, during the test, the cluster is completely idling. And it
is obvious that this test does not saturate the loading of both network
and MDT / OST hardware (they have almost no loading).
I am wondering whether this could be a bug to report to the development
team.
Best Regards,
T.H.Hsieh
On Thu, Oct 29, 2020 at 09:49:42AM -0400, Simon Guilbault wrote:
> Our current workaround was to use the following command on the MGS with
> Lustre 2.12.5 that include the patches in LU-12651 and LU-12759 (we were
> using a patched 2.12.4 a few months ago):
> lctl set_param -P osc.*.grant_shrink=0
>
> We could not find the root cause of the underlying problem, dynamic grant
> shrinking seems to be useful when the OSTs are running out of free space.
>
> On Wed, Oct 28, 2020 at 11:47 PM Tung-Han Hsieh <
> thhsieh at twcp1.phys.ntu.edu.tw> wrote:
>
> > Dear Simon,
> >
> > Thank you very much for your hint. Yes, you are right. We compared
> > the grant size of two client by (running in each client):
> >
> > lctl get_param osc.*.cur_grant_bytes
> >
> > - Client A: It has run the following large data transfer for over 36 hrs.
> >
> > while [ 1 ]; do
> > tar cf - /home/large/data | ssh remote_host "cat > /dev/null"
> > done
> >
> > The value of "cur_grant_bytes" is 796134.
> >
> > - Client B: It is almost idling during the action of Client A.
> >
> > The value of "cur_grant_bytes" is 1715863552.
> >
> > If this is the reason that hit the I/O performance of Client A seriously,
> > is it possible to maintain it at a constant value at least for the head
> > node (since the head node is the most probable one to have large and long
> > time data I/O of the whole cluster, especially for a data center) ?
> >
> > I would be also like to ask: Why this value has to be dynamically adjusted
> > ?
> >
> > Thank you very much for your comment in advance.
> >
> > Best Regards,
> >
> > T.H.Hsieh
> >
> > On Wed, Oct 28, 2020 at 02:00:21PM -0400, Simon Guilbault wrote:
> > > Hi, we had a similar performance problem on our login/DTNs node a few
> > > months ago, the problem was the grant size was shrinking and was getting
> > > stuck under 1MB. Once under 1MB, the client had to send every request to
> > > the OST using sync IO.
> > >
> > > Check the output of the following command:
> > > lctl get_param osc.*.cur_grant_bytes
> > >
> > > On Wed, Oct 28, 2020 at 12:08 AM Tung-Han Hsieh <
> > > thhsieh at twcp1.phys.ntu.edu.tw> wrote:
> > >
> > > > Dear All,
> > > >
> > > > Sorry that I am not sure whether this mail was successfully posted to
> > > > the lustre-discuss mailing list or not. So I resent it again. Please
> > > > ignore it if you already read it before.
> > > >
> > > >
> > ===========================================================================
> > > >
> > > > Dear Andreas,
> > > >
> > > > Thank you very much for your kindly suggestions. These days I got a
> > chance
> > > > to follow your suggestions for the test. This email is to report the
> > > > results
> > > > I have done so far. What I have done were:
> > > >
> > > > 1. Upgrade one client (with Infiniband) to Lustre 2.13.56_44_gf8a8d3f
> > > > (obtained from github). The compiling information is:
> > > >
> > > > - Linux kernel 4.19.123.
> > > > - Infiniband MLNX_OFED_SRC-4.6-1.0.1.1.
> > > > - ./configure --prefix=/opt/lustre \
> > > > --with-o2ib=/path/of/mlnx-ofed-kernel-4.6 \
> > > > --disable-server --enable-mpitests=no
> > > > - make
> > > > - make install
> > > >
> > > > 2. We mounted the lustre file system (lustre MDT/OST servers: version
> > > > 2.12.4 with Infiniband with ZFS backend) by this command:
> > > >
> > > > - mount -t lustre -o flock mdt at o2ib:/chome /home
> > > >
> > > > 3. The script to simulate large data transfer is following:
> > > > (the directory "/home/large/data" contains 758 files, each size
> > 600MB)
> > > >
> > > > while [ 1 ]; do
> > > > tar cf - /home/large/data | ssh remote_host "cat > /dev/null"
> > > > done
> > > >
> > > > ps. Note that this scenario is common in a large data center, while
> > > > some users transferring large data out of the data center
> > through
> > > > the head node; while other users might copy files and do their
> > > > normal works in the same head node.
> > > >
> > > > 4. During the data transfer in the background, I occationally ran this
> > > > command in the same client to test whether there is any abnormality
> > > > in I/O performance (where /home/dir1/file has size 600MB):
> > > >
> > > > cp /home/dir1/file /home/dir2/
> > > >
> > > > In the beginning this command can complete in about 1 sec. But after
> > > > around 18 hours (not exactly, because the test ran overnight while
> > > > I was sleeping), the problem appeared. The time to complete the same
> > > > cp command was more than 1 minute.
> > > >
> > > > During the test, I am sure that the whole cluster was idling. The
> > MDT
> > > > and OST servers did not have other loading. The CPU usage of the
> > testing
> > > > client was below 0.3.
> > > >
> > > > Then I stopped the test, and let the whole system completely idle.
> > But
> > > > after 3 hours, the I/O abnormality of the same "cp" command was
> > still
> > > > there. Only after I unmounted /home and remounted /home, the
> > abnormality
> > > > of "cp" recovered to normal.
> > > >
> > > > Before and after remounting /home (which I call "reset"), I did the
> > > > following tests:
> > > >
> > > > 1. Using "top" to check the memory usage:
> > > >
> > > > Before reset:
> > > > =====================================
> > > > top - 10:43:15 up 35 days, 52 min, 3 users, load average: 0.00, 0.00,
> > > > 0.00
> > > > Tasks: 404 total, 1 running, 162 sleeping, 0 stopped, 0 zombie
> > > > %Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si,
> > > > 0.0 st
> > > > KiB Mem : 13232632+total, 13000131+free, 647784 used, 1677220
> > buff/cache
> > > > KiB Swap: 15631240 total, 15631240 free, 0 used. 13076376+avail
> > Mem
> > > >
> > > > After reset:
> > > > =====================================
> > > > top - 10:48:02 up 35 days, 57 min, 3 users, load average: 0.04, 0.01,
> > > > 0.00
> > > > Tasks: 395 total, 1 running, 159 sleeping, 0 stopped, 0 zombie
> > > > %Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si,
> > > > 0.0 st
> > > > KiB Mem : 13232632+total, 12946539+free, 675948 used, 2184976
> > buff/cache
> > > > KiB Swap: 15631240 total, 15631240 free, 0 used. 13073571+avail
> > Mem
> > > >
> > > > It seems that most of the memory were in "free" state. The amount of
> > > > hidden memory was neglectable. So I did not further investigate the
> > > > amount of slab memory.
> > > >
> > > > 2. Using "strace" with the following commands:
> > > >
> > > > - Before reset (took 1 min of each cp):
> > > > strace -c -o /tmp/log2-err.txt cp /home/dir1/file /home/dir2/
> > > >
> > > > - After reset (took 1 sec of each cp):
> > > > strace -c -o /tmp/log2-reset.txt cp /home/dir1/file /home/dir2/
> > > >
> > > > From the log files, the major time consuming was read and write
> > > > syscalls.
> > > > The others are neglectable.
> > > >
> > > > % time seconds usecs/call calls errors syscall
> > > > ------ ----------- ----------- --------- --------- ----------------
> > > > (Before reset)
> > > > 71.46 0.278424 1920 145 write
> > > > 28.06 0.109322 705 155 read
> > > > (After reset)
> > > > 52.92 0.299091 2063 145 write
> > > > 46.85 0.264777 1708 155 read
> > > >
> > > > Before reset, since we have done the cp test for the same file a
> > > > few times, the file was already cached. So the reading time is
> > > > smaller before reset than that after reset (since after reset /home
> > > > was remounted).
> > > >
> > > > Hence from this result, the time of syscalls looks normal. The
> > > > performance drop seems occuring in other places.
> > > >
> > > > Now I haven't done the investigation of Lustre kernel debug log by
> > enabling
> > > > Lustre debug=-1. We will find another chance to do it.
> > > >
> > > > Up to now, any comments or suggestions are very welcome.
> > > >
> > > > Thanks for your help in advance.
> > > >
> > > >
> > > > Best Regards,
> > > >
> > > > T.H.Hsieh
> > > >
> > > >
> > > > On Thu, Oct 08, 2020 at 01:32:53PM -0600, Andreas Dilger wrote:
> > > > > On Oct 8, 2020, at 10:37 AM, Tung-Han Hsieh <
> > > > thhsieh at twcp1.phys.ntu.edu.tw> wrote:
> > > > > >
> > > > > > Dear All,
> > > > > >
> > > > > > In the past months, we encountered several times of Lustre I/O
> > > > abnormally
> > > > > > slowing down. It is quite mysterious that there seems no problem
> > on the
> > > > > > network hardware, nor the lustre itself since there is no error
> > message
> > > > > > at all in MDT/OST/client sides.
> > > > > >
> > > > > > Recently we probably found a way to reproduce it, and then have
> > some
> > > > > > suspections. We found that if we continuously perform I/O on a
> > client
> > > > > > without stop, then after some time threshold (probably more than 24
> > > > > > hours), the additional file I/O bandwidth of that client will be
> > > > shriked
> > > > > > dramatically.
> > > > > >
> > > > > > Our configuration is the following:
> > > > > > - One MDT and one OST server, based on ZFS + Lustre-2.12.4.
> > > > > > - The OST is served by a RAID 5 system with 15 SAS hard disks.
> > > > > > - Some clients connect to MDT/OST through Infiniband, some through
> > > > > > gigabit ethernet.
> > > > > >
> > > > > > Our test was focused on the clients using infiniband, which is
> > > > described
> > > > > > in the following:
> > > > > >
> > > > > > We have a huge (several TB) amount of data stored in the Lustre
> > file
> > > > > > system to be transferred to outside network. In order not to
> > exhaust
> > > > > > the network bandwidth of our institute, we transfer the data with
> > > > limited
> > > > > > bandwidth via the following command:
> > > > > >
> > > > > > rsync -av --bwlimit=1000 <data_in_Lustre>
> > > > <out_side_server>:/<out_side_path>/
> > > > > >
> > > > > > That is, the transferring rate is 1 MB per second, which is
> > relatively
> > > > > > low. The client read the data from Lustre through infiniband. So
> > during
> > > > > > data transmission, presumably there is no problem to do other data
> > I/O
> > > > > > on the same client. On average, when copy a 600 MB file from one
> > > > directory
> > > > > > to another directory (both in the same Lustre file system), it took
> > > > about
> > > > > > 1.0 - 2.0 secs, even when the rsync process still working.
> > > > > >
> > > > > > But after about 24 hours of continuously sending data via rsync,
> > the
> > > > > > additional I/O on the same client was dramatically shrinked. When
> > it
> > > > happens,
> > > > > > it took more than 1 minute to copy a 600 MB from somewhere to
> > another
> > > > place
> > > > > > (both in the same Lustre) while rsync is still running.
> > > > > >
> > > > > > Then, we stopped the rsync process, and wait for a while (about one
> > > > > > hour). The I/O performance of copying that 600 MB file returns
> > normal.
> > > > > >
> > > > > > Based on this observation, we are suspecting that whether there is
> > a
> > > > > > hidden QoS mechanism built in Lustre ? When a process occupies the
> > I/O
> > > > > > bandwidth for a long time and exceeded some limits, does Lustre
> > > > automatically
> > > > > > shrinked the I/O bandwidth for all processes running in the same
> > > > client ?
> > > > > >
> > > > > > I am not against such QoS design, if it does exist. But the amount
> > of
> > > > > > shrinking seems to be too large for infiniband (QDR and above).
> > Then
> > > > > > I am further suspecting that whether this is due to that our
> > system is
> > > > > > mixed with clients in which some have infiniband but some do not ?
> > > > > >
> > > > > > Could anyone help to fix this problem ? Any suggestions will be
> > very
> > > > > > appreciated.
> > > > >
> > > > > There is no "hidden QOS", unless it is so well hidden that I don't
> > know
> > > > > about it.
> > > > >
> > > > > You could investigate several different things to isolate the
> > problem:
> > > > > - try with a 2.13.56 client to see if the problem is already fixed
> > > > > - check if the client is using a lot of CPU when it becomes slow
> > > > > - run strace on your copy process to see which syscalls are slow
> > > > > - check memory/slab usage
> > > > > - enable Lustre debug=-1 and dump the kernel debug log to see where
> > > > > the process is taking a long time to complete a request
> > > > >
> > > > > It is definitely possible that there is some kind of problem, since
> > this
> > > > > is not a very common workload to be continuously writing to the same
> > file
> > > > > descriptor for over a day. You'll have to do the investigation on
> > your
> > > > > system to isolate the source of the problem.
> > > > >
> > > > > Cheers, Andreas
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > >
> > > >
> > > > _______________________________________________
> > > > lustre-discuss mailing list
> > > > lustre-discuss at lists.lustre.org
> > > > http://lists.lustre.org/listinfo.cgi/lustre-discuss-lustre.org
> > > >
> >
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