答复: 答复: 答复: [Xen-devel] Credit scheduler vs SEDF scheduler

["Zhiyuan Shao" <zyshao.maillist@gmail.com>]

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Some points I think that may help to your doubts:

1) how the scheduler of guest OS and VMM cooperate together.

Frankly, I also think this is a rather interesting topic to research, and
currently working on that. However, before putting hands on this topic, we
need some answers on several other tightly related problems. First problem
is time keeping mechanism of Xen VMM. Not yet reading all the related codes
of Xen, I guess for PV guests (do not consider the HVM case, since it is
more complicated than PV), each time that a VCPU is scheduled in, one of the
members of VCPU structure that stores current time will be updated (see
struct vcpu_time_info), while the time precedes when the VCPU is actually is
scheduled-out is simply “lost”. This will definitely cause problems by
confusing the scheduler of the guest OS, however, hardly find documents that
elaborate this problem or analyze what will happen in deep. 

The second problem is load-balancing. Unfortunately, this is a more
complicated problem. The guest OS (for the SMP guest cases) has its own load
balancing strategy, and I think the bottom line is that it will not waste
processing time. Taking an example here, running a single thread process
(such as “dd if=/dev/zero of=./test.file bs=64 count=3200K”, it writes a
200MB file to virtual disk) inside a two-vcpu PV guest. The process
originally running on top of single VCPU, but after a while, the VCPU is
scheduled out (blocked and waiting for the data actually write to the
virtual disk file), and another VCPU is scheduled in, the process will
migrate to the other VCPU naturally. This case will result in extra
overhead, and I got performance data to prove this. Besides understanding
the load balancing strategy of guest OS, load balancing for scheduler of the
VMM deserves also careful consideration and design. Current default
scheduler of Credit actually do not doing very well on this point. The PCPU
will steal VCPU from its neighbors once it found it is idle. Smarter
solutions are needed in later schedulers (as George had mentioned).

2) what types of events are ocurring during scheduling

I think it is good for you to check the xentrace data for this problem.

3) xenmon problems.

I had not used that and can not give you any suggestions, sorry for that.

4) Xentop

Do not know what you mean for N/W?

 

Best,

Zhiyuan

 

 

 

  _____  

发件人: xen-devel-bounces@lists.xensource.com
[mailto:xen-devel-bounces@lists.xensource.com] 代表 gaurav somani
发送时间: 2009年5月10日 20:48
收件人: xen-devel@lists.xensource.com
主题: Re: 答复: 答复: [Xen-devel] Credit scheduler vs SEDF scheduler

 

Hi list,

Some doubts.
(1) How the guest kernel scheduling strategies (linux kernel scheduling like
DEADLINE and IO schedulers in guests) affect the VM Scheduling?

(2) I want to know what types of events are ocurring during scheduling-
which domain is scheduled and what types of tasks it is running. 

I am using xenmon-   some probs.

(1) it does not show me --iocount option correctly. it always shows iocount=
0, although i am running dd utility in one of the guset domain.
(2) when using xentop - it does not show any N/W data for domain 0.

am i missing something in configuration.




2009/5/7 Zhiyuan Shao <zyshao.maillist@gmail.com>

Believe your can use BCredit for this job. However, you can only set the
attributes of a domain statically, which means the scheduler can not adjust
its behavior according to the types of the work loads. 

 

Remember there is some publication in proceedings of VEE 2009, which had
addressed your problem. The solution continuously guests the performance
characteristics of the application, and adjust the preemption decisions of
the scheduler itself. You can take a look at that, but unfortunately, they
did not provide the patch behind the solution. Besides, I am not sure that
whether such solution is really helpful to solve real problems on real
hardware, since it is based its decisions on possibilities.

 

Zhiyuan

 

  _____  

发件人: gaurav somani [mailto:onlineengineer@gmail.com] 
发送时间: 2009年5月6日 17:09
收件人: Zhiyuan Shao
抄送: xen-devel@lists.xensource.com
主题: Re: 答复: [Xen-devel] Credit scheduler vs SEDF scheduler

 

Hi Zhiyuan and George,

Thanks for the comments.

you are right i was getting spikes with more than 100ms and some times
400ms.

I will be trying with the patch u suggetsed.

one more aspect, can we classify schedulers according to applications they
run.?

Like compute intensive on Credit and Latency intensive on sEDF or some trade
off between them (In a cluster of servers).


Thanks

Gaurav

2009/5/6 Zhiyuan Shao <zyshao.maillist@gmail.com>

Hi,

 

Your experiment results are actually “expected”. There are several reasons
make the results happen. 

 

Firstly, Credit has long time slice (i.e., 30 millisec by default), and this
make the observed close-to 30ms RTT to happen, actually, during such long
RTT (the spike), the domain in test (domain 1 of your experiment) is just
scheduled-out (actually, I guess the spikes illustrated in your figure 1
should read higher than 30ms!). At the same time, the scheduled-in vcpus
(may executing the TEST program, which is cpu-hogs), and they will not give
up their 30-ms time slice if they are not preempted. 

 

Secondly, as regulated by Credit scheduler, if a VCPU has no more credits,
it will not be BOOSTed even it do have some I/O requests to handle. This
explains why your figure1, which has little spikes in the first 150 second,
and crowed spikes follow. Since vcpu of domain 1 simply used out their
credits, and can not preempt the other running vcpus of other domains, even
they do have I/O operations to accomplish (i.e., PING). 

 

Lastly, compared with Credit, SEDF generally has short time slices, and it
has bigger chances to preempt current running vcpu when another vcpu found
it has some I/O operations to be accomplished. (actually, I donot understand
SEDF very well, but conclude these features by reading the trace data)

 

Actually, you can use BCredit patch to eliminate the spikes, I can guarantee
that it will work very well! :-) And I agree with George that you should not
use PING to justify whether a scheduler is good or not.

 

Best,

Zhiyuan

 

 

  _____  

发件人: xen-devel-bounces@lists.xensource.com
[mailto:xen-devel-bounces@lists.xensource.com] 代表 gaurav somani
发送时间: 2009年5月5日 17:26
收件人: xen-devel@lists.xensource.com
主题: [Xen-devel] Credit scheduler vs SEDF scheduler

 

Hi list,

I am evaluating the scheduler behavior in xen.

I am using Xen 3.3.0 
Dom0 and Dom1,2,3 and 4 all are opensuse 11.
I have one CPU Intensive TEST which has no. of arithmatic instruction in an
infinite while() loop.
i am pinging domain1 with an external machine. and noting the RTT values.

i have below experiments
time (s)          domain state
0                 dom0,1,2,3,4 all idle
50               dom2 TEST started
100             dom3 TEST started
150             dom4 TEST started
200             dom0 TEST started
250             dom2 TEST stopped
300             dom3 TEST stopped
350             dom4 TEST stopped
400             dom0 TEST stopped

For these 400 seconds time, i have performed experiments with Credit and
SEDF sceduler.
the configuration is 


Credit configuration - weight 256, cap 0
Domain                  VCPU 
0                                  2
1                                  2
2                                  2
3                                  2
4                                  2
all vcpu0s are pinned to pcpu0 and vcpu1s to pcpu1.


SEDF configuration - Period 10ms slice - 1.9ms
Domain                  VCPU 
0                                  2
1                                  2
2                                  2
3                                  2
4                                  2
all vcpu0s are pinned to pcpu0 and vcpu1s to pcpu1.

the results of RTT values are attached herewith. the performance of Credit
is very bad in comparison to SEDF in this scenario.
Please provide me some thought on it.


Thanks and Regards

Gaurav somani
M.Tech (ICT)
Dhribhai Ambani Institute of ICT,
INDIA

http://dcomp.daiict.ac.in/~gaurav <http://dcomp.daiict.ac.in/%7Egaurav> 








-- 
Gaurav somani
M.Tech (ICT)
Dhribhai Ambani Institute of ICT,
Gandhinagar
Gujarat, INDIA

onlineengineer@ieee.org
gaurav_somani@daiict.ac.in
http://dcomp.daiict.ac.in/~gaurav <http://dcomp.daiict.ac.in/%7Egaurav> 
http://www.linkedin.com/in/gauravsomani




-- 
Gaurav somani
M.Tech (ICT)
Dhribhai Ambani Institute of ICT,
Gandhinagar
Gujarat, INDIA

onlineengineer@ieee.org
gaurav_somani@daiict.ac.in
http://dcomp.daiict.ac.in/~gaurav
http://www.linkedin.com/in/gauravsomani


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