Re: [Qemu-devel] What's the advantages of POSTCOPY over CPU-THROTTLE?

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: "Zhangbo (Oscar)" <oscar.zhangbo@huawei.com>
Cc: "zhouyimin Zhou(Yimin)" <zhouyimin@huawei.com>,
	Zhanghailiang <zhang.zhanghailiang@huawei.com>,
	Yanqiangjun <yanqiangjun@huawei.com>,
	"Huangpeng (Peter)" <peter.huangpeng@huawei.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"Herongguang (Stephen)" <herongguang.he@huawei.com>,
	Linqiangmin <linqiangmin@huawei.com>,
	Huangzhichao <huangzhichao@huawei.com>,
	"Wangyufei (James)" <james.wangyufei@huawei.com>
Subject: Re: [Qemu-devel] What's the advantages of POSTCOPY over CPU-THROTTLE?
Date: Wed, 6 Jan 2016 09:57:32 +0000	[thread overview]
Message-ID: <20160106095731.GB2528@work-vm> (raw)
In-Reply-To: <0259E1C966E8C54AA93AA2B1240828E650F34EC8@szxema507-mbs.china.huawei.com>

* Zhangbo (Oscar) (oscar.zhangbo@huawei.com) wrote:
> Hi all:
> 　　Postcopy is suitable for migrating guests which have large page change rates. It 
>     1 makes the guest run at the destination ASAP.
>     2 makes the downtime of the guest small enough.
>     If we don't take the 1st advantage into account, then, its benefit seems similar with CPU-THROTTLE: both of them make the guest's downtime small during migration.
>  
>     CPU-THROTTLE would make the guest's dirtypage rate *smaller than the network bandwidth*, in order to make the to_send_page_number in each iteration convergent and achieve the small-enough downtime during the last iteration.
>     If we adopt POST-COPY here, the guest's dirtypage rate would *become equal to the bandwidth*, because we have to fetch its memory from the source side, via the network.
>     Both of them would introduce performance degradations of the guest, which may in turn cause downtime larger.
> 
>     So, here comes the question: If we just compare POSTCOPY with CPU-THROTTLE for their advantages in decreasing downtime, POSTCOPY seems has no pos over CPU-THROTTLE, is that right?
> 
>     Meanwhile, Are there any other benifits of POSTCOPY besides the 2 mentioned above?

It's a good question and they do both try and help solve the same problem.
One problem with cpu-throttle is whether you can throttle the CPU enough to
get the dirty-rate below the rate of the network, and the answer to that is
very workload dependent.  On a large, many-core VM, even a little bit of CPU
can dirty a lot of memory.  Postcopy is guaranteed to finish migration,
irrespective of the workload.

Postcopy is pretty fine-grained, in that only threads that are accessing
pages that are still on the source are blocked, since it allows the use
of async page faults, that means it's even finer grained than the vCPU level,
so many threads come back up to full performance pretty quickly
even if there are a few pages left.

Dave

> 
> Oscar
> 
>     
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK