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

[Wei Yang]

On Thu, Mar 24, 2016 at 01:32:25AM +0000, Li, Liang Z wrote:
>> >> >> >
>> >> >> >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 main  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.
>> >
>> 
>> My poor knowledge~ Should google it before I imagine the meaning of the
>> terminology.
>> 
>> If my understanding is correct, the Page Cache is counted as Free Page, while
>> actually we should migrate them instead of filter them.
>
>No, the Page Cache is not counted as Free Page ...

OK, I misunderstand the concept in wiki.

The Page Cache is a trade off between Free Page percentage and the I/O
performance in guest.

>
>Liang

-- 
Richard Yang\nHelp you, Help me