Re: Impact of page cache on OSD read performance for SSD

Mark Nelson <mark.nelson@xxxxxxxxxxx> · Wed, 24 Sep 2014 08:27:40 -0500

On 09/24/2014 07:38 AM, Sage Weil wrote:
On Wed, 24 Sep 2014, Haomai Wang wrote:
I agree with that direct read will help for disk read. But if read data
is hot and small enough to fit in memory, page cache is a good place to
hold data cache. If discard page cache, we need to implement a cache to
provide with effective lookup impl.

This is true for some workloads, but not necessarily true for all.  Many
clients (notably RBD) will be caching at the client side (in VM's fs, and
possibly in librbd itself) such that caching at the OSD is largely wasted
effort.  For RGW the often is likely true, unless there is a varnish cache
or something in front.

We should probably have a direct_io config option for filestore.  But even
better would be some hint from the client about whether it is caching or
not so that FileStore could conditionally cache...

I like the hinting idea.  Having said that, if the effect being seen is 
due to page cache, it seems like something is off.  We've seen 
performance issues in the kernel before so it's not unprecedented. 
Working around it with direct IO could be the right way to go, but it 
might be that this is something that could be fixed higher up and 
improve performance in other scenarios too.  I'd hate to let it go by 
the wayside of we could find something actionable.

sage

  >
BTW, whether to use direct io we can refer to MySQL Innodb engine with
direct io and PostgreSQL with page cache.

On Wed, Sep 24, 2014 at 10:29 AM, Somnath Roy <Somnath.Roy@xxxxxxxxxxx> wrote:
Haomai,
I am considering only about random reads and the changes I made only affecting reads. For write, I have not measured yet. But, yes, page cache may be helpful for write coalescing. Still need to evaluate how it is behaving comparing direct_io on SSD though. I think Ceph code path will be much shorter if we use direct_io in the write path where it is actually executing the transactions. Probably, the sync thread and all will not be needed.

I am trying to analyze where is the extra reads coming from in case of buffered io by using blktrace etc. This should give us a clear understanding what exactly is going on there and it may turn out that tuning kernel parameters only  we can achieve similar performance as direct_io.

Thanks & Regards
Somnath

-----Original Message-----
From: Haomai Wang [mailto:haomaiwang@xxxxxxxxx]
Sent: Tuesday, September 23, 2014 7:07 PM
To: Sage Weil
Cc: Somnath Roy; Milosz Tanski; ceph-devel@xxxxxxxxxxxxxxx
Subject: Re: Impact of page cache on OSD read performance for SSD

Good point, but do you have considered that the impaction for write ops? And if skip page cache, FileStore is responsible for data cache?

On Wed, Sep 24, 2014 at 3:29 AM, Sage Weil <sweil@xxxxxxxxxx> wrote:
On Tue, 23 Sep 2014, Somnath Roy wrote:
Milosz,
Thanks for the response. I will see if I can get any information out of perf.

Here is my OS information.

root@emsclient:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 13.10
Release:        13.10
Codename:       saucy
root@emsclient:~# uname -a
Linux emsclient 3.11.0-12-generic #19-Ubuntu SMP Wed Oct 9 16:20:46
UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

BTW, it's not a 45% drop, as you can see, by tuning the OSD parameter I was able to get almost *2X* performance improvement with direct_io.
It's not only page cache (memory) lookup, in case of buffered_io  the following could be problem.

1. Double copy (disk -> file buffer cache, file buffer cache -> user
buffer)

2. As the iostat output shows, it is not reading 4K only, it is
reading more data from disk as required and in the end it will be
wasted in case of random workload..

It might be worth using blktrace to see what the IOs it is issueing are.
Which ones are > 4K and what they point to...

sage

Thanks & Regards
Somnath

-----Original Message-----
From: Milosz Tanski [mailto:milosz@xxxxxxxxx]
Sent: Tuesday, September 23, 2014 12:09 PM
To: Somnath Roy
Cc: ceph-devel@xxxxxxxxxxxxxxx
Subject: Re: Impact of page cache on OSD read performance for SSD

Somnath,

I wonder if there's a bottleneck or a point of contention for the kernel. For a entirely uncached workload I expect the page cache lookup to cause a slow down (since the lookup should be wasted). What I wouldn't expect is a 45% performance drop. Memory speed should be one magnitude faster then a modern SATA SSD drive (so it should be more negligible overhead).

Is there anyway you could perform the same test but monitor what's going on with the OSD process using the perf tool? Whatever is the default cpu time spent hardware counter is fine. Make sure you have the kernel debug info package installed so can get symbol information for kernel and module calls. With any luck the diff in perf output in two runs will show us the culprit.

Also, can you tell us what OS/kernel version you're using on the OSD machines?

- Milosz

On Tue, Sep 23, 2014 at 2:05 PM, Somnath Roy <Somnath.Roy@xxxxxxxxxxx> wrote:
Hi Sage,
I have created the following setup in order to examine how a single OSD is behaving if say ~80-90% of ios hitting the SSDs.

My test includes the following steps.

         1. Created a single OSD cluster.
         2. Created two rbd images (110GB each) on 2 different pools.
         3. Populated entire image, so my working set is ~210GB. My system memory is ~16GB.
         4. Dumped page cache before every run.
         5. Ran fio_rbd (QD 32, 8 instances) in parallel on these two images.

Here is my disk iops/bandwidth..

         root@emsclient:~/fio_test# fio rad_resd_disk.job
         random-reads: (g=0): rw=randread, bs=4K-4K/4K-4K, ioengine=libaio, iodepth=64
         2.0.8
         Starting 1 process
         Jobs: 1 (f=1): [r] [100.0% done] [154.1M/0K /s] [39.7K/0  iops] [eta 00m:00s]
         random-reads: (groupid=0, jobs=1): err= 0: pid=1431
         read : io=9316.4MB, bw=158994KB/s, iops=39748 , runt=
60002msec

My fio_rbd config..

[global]
ioengine=rbd
clientname=admin
pool=rbd1
rbdname=ceph_regression_test1
invalidate=0    # mandatory
rw=randread
bs=4k
direct=1
time_based
runtime=2m
size=109G
numjobs=8
[rbd_iodepth32]
iodepth=32

Now, I have run Giant Ceph on top of that..

1. OSD config with 25 shards/1 thread per shard :
-------------------------------------------------------

          avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           22.04    0.00   16.46   45.86    0.00   15.64

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     9.00    0.00    6.00     0.00    92.00    30.67     0.01    1.33    0.00    1.33   1.33   0.80
sdd               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sde               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdg               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdf               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdh             181.00     0.00 34961.00    0.00 176740.00     0.00    10.11   102.71    2.92    2.92    0.00   0.03 100.00
sdc               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdb               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

ceph -s:
  ----------
root@emsclient:~# ceph -s
     cluster 94991097-7638-4240-b922-f525300a9026
      health HEALTH_OK
      monmap e1: 1 mons at {a=10.196.123.24:6789/0}, election epoch 1, quorum 0 a
      osdmap e498: 1 osds: 1 up, 1 in
       pgmap v386366: 832 pgs, 7 pools, 308 GB data, 247 kobjects
             366 GB used, 1122 GB / 1489 GB avail
                  832 active+clean
   client io 75215 kB/s rd, 18803 op/s

  cpu util:
----------
  Gradually decreases from ~21 core (serving from cache) to ~10 core (while serving from disks).

  My Analysis:
-----------------
  In this case "All is Well"  till ios are served from cache (XFS is
smart enough to cache some data ) . Once started hitting disks and throughput is decreasing. As you can see, disk is giving ~35K iops , but, OSD throughput is only ~18.8K ! So, cache miss in case of buffered io seems to be very  expensive.  Half of the iops are waste. Also, looking at the bandwidth, it is obvious, not everything is 4K read, May be kernel read_ahead is kicking (?).

Now, I thought of making ceph disk read as direct_io and do the same experiment. I have changed the FileStore::read to do the direct_io only. Rest kept as is. Here is the result with that.

Iostat:
-------

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           24.77    0.00   19.52   21.36    0.00   34.36

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdd               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sde               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdg               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdf               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdh               0.00     0.00 25295.00    0.00 101180.00     0.00     8.00    12.73    0.50    0.50    0.00   0.04 100.80
sdc               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdb               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

ceph -s:
  --------
root@emsclient:~/fio_test# ceph -s
     cluster 94991097-7638-4240-b922-f525300a9026
      health HEALTH_OK
      monmap e1: 1 mons at {a=10.196.123.24:6789/0}, election epoch 1, quorum 0 a
      osdmap e522: 1 osds: 1 up, 1 in
       pgmap v386711: 832 pgs, 7 pools, 308 GB data, 247 kobjects
             366 GB used, 1122 GB / 1489 GB avail
                  832 active+clean
   client io 100 MB/s rd, 25618 op/s

cpu util:
--------
   ~14 core while serving from disks.

  My Analysis:
  ---------------
No surprises here. Whatever is disk throughput ceph throughput is almost matching.

Let's tweak the shard/thread settings and see the impact.

2. OSD config with 36 shards and 1 thread/shard:
-----------------------------------------------------------

    Buffered read:
    ------------------
   No change, output is very similar to 25 shards.

   direct_io read:
   ------------------
        Iostat:
       ----------
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           33.33    0.00   28.22   23.11    0.00   15.34

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     0.00    0.00    2.00     0.00    12.00    12.00     0.00    0.00    0.00    0.00   0.00   0.00
sdd               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sde               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdg               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdf               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdh               0.00     0.00 31987.00    0.00 127948.00     0.00     8.00    18.06    0.56    0.56    0.00   0.03 100.40
sdc               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdb               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

        ceph -s:
     --------------
root@emsclient:~/fio_test# ceph -s
     cluster 94991097-7638-4240-b922-f525300a9026
      health HEALTH_OK
      monmap e1: 1 mons at {a=10.196.123.24:6789/0}, election epoch 1, quorum 0 a
      osdmap e525: 1 osds: 1 up, 1 in
       pgmap v386746: 832 pgs, 7 pools, 308 GB data, 247 kobjects
             366 GB used, 1122 GB / 1489 GB avail
                  832 active+clean
   client io 127 MB/s rd, 32763 op/s

         cpu util:
    --------------
        ~19 core while serving from disks.

          Analysis:
------------------
         It is scaling with increased number of shards/threads. The parallelism also increased significantly.

3. OSD config with 48 shards and 1 thread/shard:
  ----------------------------------------------------------
     Buffered read:
    -------------------
     No change, output is very similar to 25 shards.

    direct_io read:
     -----------------
        Iostat:
       --------

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           37.50    0.00   33.72   20.03    0.00    8.75

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdd               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sde               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdg               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdf               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdh               0.00     0.00 35360.00    0.00 141440.00     0.00     8.00    22.25    0.62    0.62    0.00   0.03 100.40
sdc               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdb               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

          ceph -s:
        --------------
root@emsclient:~/fio_test# ceph -s
     cluster 94991097-7638-4240-b922-f525300a9026
      health HEALTH_OK
      monmap e1: 1 mons at {a=10.196.123.24:6789/0}, election epoch 1, quorum 0 a
      osdmap e534: 1 osds: 1 up, 1 in
       pgmap v386830: 832 pgs, 7 pools, 308 GB data, 247 kobjects
             366 GB used, 1122 GB / 1489 GB avail
                  832 active+clean
   client io 138 MB/s rd, 35582 op/s

          cpu util:
  ----------------
         ~22.5 core while serving from disks.

           Analysis:
  --------------------
         It is scaling with increased number of shards/threads. The parallelism also increased significantly.

4. OSD config with 64 shards and 1 thread/shard:
  ---------------------------------------------------------
       Buffered read:
      ------------------
      No change, output is very similar to 25 shards.

      direct_io read:
      -------------------
        Iostat:
       ---------
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           40.18    0.00   34.84   19.81    0.00    5.18

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdd               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sde               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdg               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdf               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdh               0.00     0.00 39114.00    0.00 156460.00     0.00     8.00    35.58    0.90    0.90    0.00   0.03 100.40
sdc               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdb               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

        ceph -s:
  ---------------
root@emsclient:~/fio_test# ceph -s
     cluster 94991097-7638-4240-b922-f525300a9026
      health HEALTH_OK
      monmap e1: 1 mons at {a=10.196.123.24:6789/0}, election epoch 1, quorum 0 a
      osdmap e537: 1 osds: 1 up, 1 in
       pgmap v386865: 832 pgs, 7 pools, 308 GB data, 247 kobjects
             366 GB used, 1122 GB / 1489 GB avail
                  832 active+clean
   client io 153 MB/s rd, 39172 op/s

       cpu util:
----------------
     ~24.5 core while serving from disks. ~3% cpu left.

        Analysis:
------------------
       It is scaling with increased number of shards/threads. The parallelism also increased significantly. It is disk bound now.

Summary:

So, it seems buffered IO has significant impact on performance in case backend is SSD.
My question is,  if the workload is very random and storage(SSD) is very huge compare to system memory, shouldn't we always go for direct_io instead of buffered io from Ceph ?

Please share your thoughts/suggestion on this.

Thanks & Regards
Somnath

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