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Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping fre

From: Li, Liang Z
Subject: Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping free pages
Date: Wed, 23 Mar 2016 14:35:42 +0000

> >No special purpose. Maybe it's caused by the email client. I didn't
> >find the character in the original doc.
> >
> https://lists.gnu.org/archive/html/qemu-devel/2016-03/msg00715.html
> You could take a look at this link, there is a '>' before From.

Yes, there is. 

> >> >
> >> >6. Handling page cache in the guest
> >> >The memory used for page cache in the guest will change depends on
> >> >the workload, if guest run some block IO intensive work load, there
> >> >will
> >>
> >> Would this improvement benefit a lot when guest only has little free page?
> >
> >Yes, the improvement is very obvious.
> >
> Good to know this.
> >> In your Performance data Case 2, I think it mimic this kind of case.
> >> While the memory consuming task is stopped before migration. If it
> >> continues, would we still perform better than before?
> >
> >Actually, my RFC patch didn't consider the page cache, Roman raised this
> issue.
> >so I add this part in this doc.
> >
> >Case 2 didn't mimic this kind of scenario, the work load is an memory
> >consuming work load, not an block IO intensive work load, so there are
> >not many page cache in this case.
> >
> >If the work load in case 2 continues, as long as it not write all the
> >memory it allocates, we still can get benefits.
> >
> Sounds I have little knowledge on page cache, and its relationship between
> free page and I/O intensive work.
> Here is some personal understanding, I would appreciate if you could correct
> me.
>                 +---------+
>                 |PageCache|
>                 +---------+
>       +---------+---------+---------+---------+
>       |Page     |Page     |Free Page|Page     |
>       +---------+---------+---------+---------+
> Free Page is a page in the free_list, PageCache is some page cached in CPU's
> cache line?

No, page cache is quite different with CPU cache line.
" In computing, a page cache, sometimes also called disk cache,[2] is a 
transparent cache
 for the pages originating from a secondary storage device such as a hard disk 
drive (HDD).
 The operating system keeps a page cache in otherwise unused portions of the 
 memory (RAM), resulting in quicker access to the contents of cached pages and 
overall performance improvements "
you can refer to https://en.wikipedia.org/wiki/Page_cache
for more details.

> When memory consuming task runs, it leads to little Free Page in the whole
> system. What's the consequence when I/O intensive work runs? I guess it
> still leads to little Free Page. And will have some problem in sync on
> PageCache?
> >>
> >> I am thinking is it possible to have a threshold or configurable
> >> threshold to utilize free page bitmap optimization?
> >>
> >
> >Could you elaborate your idea? How does it work?
> >
> Let's back to Case 2. We run a memory consuming task which will leads to
> little Free Page in the whole system. Which means from Qemu perspective,
> little of the dirty_memory is filtered by Free Page list. My original 
> question is
> whether your solution benefits in this scenario. As you mentioned it works
> fine. So maybe this threshold is not necessary.
I didn't quite understand your question before. 
The benefits we get depends on the  count of free pages we can filter out.
This is always true.

> My original idea is in Qemu we can calculate the percentage of the Free Page
> in the whole system. If it finds there is only little percentage of Free Page,
> then we don't need to bother to use this method.

I got you. The threshold can be used for optimization, but the effect is very 
If there are only a few of free pages, the process of constructing the free page
bitmap is very quick. 
But we can stop doing the following things, e.g. sending the free page bitmap 
and doing
the bitmap operation, theoretically, that may help to save some time, maybe 
several ms.

I think a VM has no free pages at all is very rare, in the worst case, there 
are still several
 MB of free pages. The proper threshold should be determined by comparing  the 
 time spends on processing the free page bitmap and the time spends on sending
the several MB of free pages though the network. If the formal is longer, we 
can stop
using this method. So we should take the network bandwidth into consideration, 
too complicated and not worth to do.


> Have a nice day~
> >Liang
> >
> >>
> >> --
> >> Richard Yang\nHelp you, Help me
> --
> Richard Yang\nHelp you, Help me

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