Re: [PATCH] io-controller: implement per group request allocationlimitation
From: Munehiro Ikeda
Date: Mon Aug 03 2009 - 22:04:01 EST
Gui Jianfeng wrote, on 07/14/2009 03:45 AM:
Munehiro Ikeda wrote:
Vivek Goyal wrote, on 07/13/2009 12:03 PM:
On Fri, Jul 10, 2009 at 09:56:21AM +0800, Gui Jianfeng wrote:
Hi Vivek,
This patch exports a cgroup based per group request limits interface.
and removes the global one. Now we can use this interface to perform
different request allocation limitation for different groups.
Thanks Gui. Few points come to mind.
- You seem to be making this as per cgroup limit on all devices. I guess
that different devices in the system can have different settings of
q->nr_requests and hence will probably want different per group limit.
So we might have to make it per cgroup per device limit.
From the viewpoint of implementation, there is a difficulty in my mind to
implement per cgroup per device limit arising from that io_group is
allocated
when associated device is firstly used. I guess Gui chose per cgroup limit
on all devices approach because of this, right?
Yes, I choose this solution from the simplicity point of view, the code will
get complicated if choosing per cgroup per device limit. But it seems per
cgroup per device limits is more reasonable.
- There does not seem to be any checks for making sure that children
cgroups don't have more request descriptors allocated than parent
group.
- I am re-thinking that what's the advantage of configuring request
descriptors also through cgroups. It does bring in additional
complexity
with it and it should justfiy the advantages. Can you think of some?
Until and unless we can come up with some significant advantages, I
will
prefer to continue to use per group limit through q->nr_group_requests
interface instead of cgroup. Once things stablize, we can revisit
it and
see how this interface can be improved.
I agree. I will try to clarify if per group per device limitation is
needed
or not (or, if it has the advantage beyond the complexity) through some
tests.
Great, hope to hear you soon.
Sorry for so late. I tried it, and write the result and my opinion
below...
Scenario
====================
The possible scenario where per-cgroup nr_requests limitation is
beneficial in my mind is that:
- Process P1 in cgroup "g1" is running with submitting many requests
to a device. The number of the requests in the device queue is
almost nr_requests for the device.
- After a while, process P2 in cgroup "g2" starts running. P2 also
tries to submit requests as many as P1.
- Assuming that user wants P2 to grab bandwidth as soon as possible
and keep it certain level.
In this scenario, I predicted the bandwidth behavior of P2 along with
tuning global nr_group_requests like below.
- If having nr_group_requests almost same as nr_requests, P1 can
allocate requests up to nr_requests and there is no room for P2 at
the beginning of its running. As a result of it, P2 has to wait
for a while till P1's requests are completed and rising of
bandwidth is delayed.
- If having nr_group_requests fewer to restrict requests from P1 and
make room for P2, the bandwidth of P2 may be lower than the case
that P1 can allocate more requests.
If the prediction is correct and per-cgroup nr_requests limitation can
make the situation better, per-cgroup nr_requests is supposed to be
beneficial.
Verification Conditions
========================
- Kernel:
2.6.31-rc1
+ Patches from Vivek on Jul 2, 2009
(IO scheduler based IO controller V6)
https://lists.linux-foundation.org/pipermail/containers/2009-July/018948.html
+ Patches from Gui Jianfeng on Jul 7, 2009 (Bug fixes)
https://lists.linux-foundation.org/pipermail/containers/2009-July/019086.html
https://lists.linux-foundation.org/pipermail/containers/2009-July/019087.html
+ Patch from Gui Jianfeng on Jul 9, 2009 (per-cgroup requests limit)
https://lists.linux-foundation.org/pipermail/containers/2009-July/019123.html
+ Patch from me on Jul 16, 2009 (Bug fix)
https://lists.linux-foundation.org/pipermail/containers/2009-July/019286.html
+ 2 local bug fix patches
(Not posted yet, I'm posting them in following mails)
- All results are measured under nr_requests=500.
- Used fio to make I/O. Job file is like below. Used libaio and
direct-I/O and tuned iodepth to make rl->count[1] approx 500 always.
----- fio job file : from here -----
[global]
size=128m
directory=/mnt/b1
runtime=30
time_based
write_bw_log
bwavgtime=200
rw=randread
direct=1
ioengine=libaio
iodepth=500
[g1]
exec_prerun=./pre.sh /mnt/cgroups/g1
exec_postrun=./log.sh /mnt/cgroups/g1 sdb "_post"
[g2]
startdelay=10
exec_prerun=./pre.sh /mnt/cgroups/g2
exec_postrun=./log.sh /mnt/cgroups/g2 sdb "_post"
----- fio job file : till here -----
Note:
pre.sh and log.sh used in exec_{pre|post}run are to assign processes
to expected cgroups and record the conditions. Let me omit the detail
of them because they are not fundamental part of this verification.
Results
====================
Bandwidth of g2 (=P2) was measured under some conditions. Conditions
and bandwidth logs are shown below.
Bandwidth logs are shown only the beginning part (from starting of P2
to 3000[ms] after aprox.) because the full logs are so long. Average
bandwidth from the beginning of log to ~10[sec] is also calculated.
Note1:
fio seems to log bandwidth only when actual data transfer occurs
(correct me if it's not true). This means that there is no line with
BW=0. In there is no data transfer, the time-stamp are simply skipped
to record.
Note2:
Graph picture of the bandwidth logs is attached.
Result(1): orange
Result(2): green
Result(3): brown
Result(4): black
---------- Result (1) ----------
* Both of g1 and g2 have nr_group_requests=500
< Conditions >
nr_requests = 500
g1/
io.nr_group_requests = 500
io.weight = 500
io.ioprio_class = 2
g2/
io.nr_group_requests = 500
io.weight = 500
io.ioprio_class = 2
< Bandwidth log of g2 >
t [ms] bw[KiB/s]
969 4
1170 1126
1374 1084
1576 876
1776 901
1980 1069
2191 1087
2400 1117
2612 1087
2822 1136
...
< Average bandwidth >
1063 [KiB/s]
(969~9979[ms])
---------- Result (2) ----------
* Both of g1 and g2 have nr_group_requests=100
< Conditions >
nr_requests = 500
g1/
io.nr_group_requests = 100
io.weight = 500
io.ioprio_class = 2
g2/
io.nr_group_requests = 100
io.weight = 500
io.ioprio_class = 2
< Bandwidth log of g2 >
t [ms] bw[KiB/s]
1498 2
1733 892
2096 722
2311 1224
2534 1180
2753 1197
2988 1137
...
< Average bandwidth >
998 [KiB/s]
(1498~9898[ms])
---------- Result (3) ----------
* To set different nr_group_requests on g1 and g2
< Conditions >
nr_requests = 500
g1/
io.nr_group_requests = 100
io.weight = 500
io.ioprio_class = 2
g2/
io.nr_group_requests = 500
io.weight = 500
io.ioprio_class = 2
< Bandwidth log of g2 >
t [ms] bw[KiB/s]
244 839
451 1133
659 964
877 1038
1088 1125
1294 979
1501 1068
1708 934
1916 1048
2117 1126
2328 1111
2533 1118
2758 1206
2969 990
...
< Average bandwidth >
1048 [KiB/s]
(244~9906[ms])
---------- Result (4) ----------
* To make g2/io.ioprio_class as RT
< Conditions >
nr_requests = 500
g1/
io.nr_group_requests = 500
io.weight = 500
io.ioprio_class = 2
g2/
io.nr_group_requests = 500
io.weight = 500
io.ioprio_class = 1
< Bandwidth log of g2 >
t [ms] bw[KiB/s]
476 8
677 2211
878 2221
1080 2486
1281 2241
1481 2109
1681 2334
1882 2129
2082 2211
2283 1915
2489 1778
2690 1915
2891 1997
...
< Average bandwidth >
2132[KiB/s]
(476~9954[ms])
Consideration and Conclusion
=============================
From result(1), it is observed that it takes 1000~1200[ms] to rise
P2 bandwidth. In result(2), where both of g1 and g2 have
nr_group_requests=100, the delay gets longer as 1800~2000[ms]. In
addition to it, the average bandwidth becomes ~5% lower than
result(1). This is supposed that P2 couldn't allocate enough requests.
Then, result(3) shows that bandwidth of P2 can rise quickly (~300[ms])
if nr_group_requests can be set per-cgroup. Result(4) shows that the
delay can be shortened by setting g2 as RT class, however, the delay
is still longer than result(3).
I think it is confirmed that "per-cgroup nr_requests limitation is
useful in a certain situation". Beyond that, the discussion topic is
the benefit pointed out above is eligible for the complication of the
implementation. IMHO, I don't think the implementation of per-cgroup
request limitation is too complicated to accept. On the other hand I
guess it suddenly gets complicated if we try to implement further
more, especially hierarchical support. It is also true that I have a
feeling that implementation without per-device limitation and
hierarchical support is like "unfinished work".
So, my opinion so far is that, per-cgroup nr_requests limitation
should be merged only if hierarchical support is concluded
"unnecessary" for it. If merging it tempts hierarchical support, it
shouldn't be.
How about your opinion, all?
My considerations or verification method might be wrong. Please
correct them if any. And if you have any other idea of scenario to
verify the effect of per-cgroup nr_requests limitation, please let me
know. I'll try it.
--
IKEDA, Munehiro
NEC Corporation of America
m-ikeda@xxxxxxxxxxxxx
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