Re: [Qemu-devel] An issue for migration determining the cpu throttle value according to workload

[Chao Fan <fanc.fnst@cn.fujitsu.com>]

ping...

Thanks,
Chao Fan

On Tue, Dec 06, 2016 at 04:52:11PM +0800, Chao Fan wrote:
>Hi all,
>
>Here is an issue in auto-converge feature of migration.
>
>When migrating a guest which consumes too much CPU & memory, dirty
>pages amount will increase significantly, so does the migration
>time, migration can not even complete, at worst.
>
>I did some simple tests on this feature. Set the two parameters
>the same as 10,20,30,40,50,60,70,80,99 and run the same task in the
>same guest. The result roughly is, with the increment of the
>two parameters, the total_time and the dirty_sync_count will decrease.
>Result shows larger the value of the two parameters is, faster the
>migration is, but much more slowly the guest runs.
>
>So I think there should be a appropriate throttle value according to
>the workload of guest. But users do not know how to determine the
>appropriate value.
>
>So I want to do a job that qemu can set the throttle value according
>to the workload of guest. I think qemu could calculate the instant
>dirty pages rate, and then determine a appropriate throttle value.
>The instant dirty pages rate means in a short fixed time, how
>many dirty pages born. But I have two questions:
>1. Where to add this feature. I have two options:
>   a. Now qemu detects the rest migration time and decides whether
>      to execute the CPU throttle. It can be changed to that qemu
>      executes the CPU throttle when instant dirty pages rate increases
>      to a certain threshold and sets the throttle value according to
>      the instant dirty pages rate. 
>   b. Using the current way as it is, when the rest migration time
>      is too long and begin to execute the CPU throttle, assign
>      appropriate throttle value according to the workload. Codes
>      will be changed fewer in this method.
>2. How to determine the CPU throttle value according to the dirty pages.
>   My preliminary idea is, the CPU throttle should be related to
>   the instant dirty pages rate and the total memory.
>   But I am not sure how to do the map from instant dirty pages rate
>   and total memory to CPU throttle value is best.
>
>Any comments will be welcome, and I want to know whether more people
>think this feature is needed.
>If anyone has good ideas, please tell me.
>
>Thanks,
>Chao Fan