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

[Li, Liang Z]

> >> >> >
> >> >> >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 ...

Liang