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Re: [Qemu-devel] [PATCH v7 0/5] virtio-balloon: free page hint reporting
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [PATCH v7 0/5] virtio-balloon: free page hint reporting support |
Date: |
Fri, 1 Jun 2018 18:02:34 +0800 |
User-agent: |
Mutt/1.9.5 (2018-04-13) |
On Fri, Jun 01, 2018 at 03:29:45PM +0800, Wei Wang wrote:
> On 06/01/2018 01:07 PM, Peter Xu wrote:
> > On Fri, Jun 01, 2018 at 12:58:24PM +0800, Peter Xu wrote:
> > > On Tue, Apr 24, 2018 at 02:13:43PM +0800, Wei Wang wrote:
> > > > This is the deivce part implementation to add a new feature,
> > > > VIRTIO_BALLOON_F_FREE_PAGE_HINT to the virtio-balloon device. The device
> > > > receives the guest free page hints from the driver and clears the
> > > > corresponding bits in the dirty bitmap, so that those free pages are
> > > > not transferred by the migration thread to the destination.
> > > >
> > > > - Test Environment
> > > > Host: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
> > > > Guest: 8G RAM, 4 vCPU
> > > > Migration setup: migrate_set_speed 100G, migrate_set_downtime 2
> > > > second
> > > >
> > > > - Test Results
> > > > - Idle Guest Live Migration Time (results are averaged over 10
> > > > runs):
> > > > - Optimization v.s. Legacy = 271ms vs 1769ms --> ~86% reduction
> > > > - Guest with Linux Compilation Workload (make bzImage -j4):
> > > > - Live Migration Time (average)
> > > > Optimization v.s. Legacy = 1265ms v.s. 2634ms --> ~51%
> > > > reduction
> > > > - Linux Compilation Time
> > > > Optimization v.s. Legacy = 4min56s v.s. 5min3s
> > > > --> no obvious difference
> > > >
> > > > - Source Code
> > > > - QEMU: https://github.com/wei-w-wang/qemu-free-page-lm.git
> > > > - Linux: https://github.com/wei-w-wang/linux-free-page-lm.git
> > > Hi, Wei,
> > >
> > > I have a very high-level question to the series.
> > >
> > > IIUC the core idea for this series is that we can avoid sending some
> > > of the pages if we know that we don't need to send them. I think this
> > > is based on the fact that on the destination side all the pages are by
> > > default zero after they are malloced. While before this series, IIUC
> > > any migration will send every single page to destination, no matter
> > > whether it's zeroed or not. So I'm uncertain about whether this will
> > > affect the received bitmap on the destination side. Say, before this
> > > series, the received bitmap will directly cover the whole RAM bitmap
> > > after migration is finished, now it's won't. Will there be any side
> > > effect? I don't see obvious issue now, but just raise this question
> > > up.
> > >
> > > Meanwhile, this reminds me about a more funny idea: whether we can
> > > just avoid sending the zero pages directly from QEMU's perspective.
> > > In other words, can we just do nothing if save_zero_page() detected
> > > that the page is zero (I guess the is_zero_range() can be fast too,
> > > but I don't know exactly how fast it is)? And how that would be
> > > differed from this page hinting way in either performance and other
> > > aspects.
> > I noticed a problem (after I wrote the above paragraph 5 minutes
> > ago...): when a page was valid and sent to the destination (with
> > non-zero data), however after a while that page was zeroed. Then if
> > we don't send zero pages at all, we won't send the page after it's
> > zeroed. Then on the destination side we'll have a stale non-zero
> > page. Is my understanding correct? Will that be a problem to this
> > series too where a valid page can be possibly freed and hinted?
>
> I think that won't be an issue either for zero page optimization or this
> free page optimization.
>
> For the zero page optimization, QEMU always sends compressed 0s to the
> destination. The zero page is detected at the time QEMU checks it (before
> sending the page). if it is a 0 page, QEMU compresses all 0s (actually just
> a flag) and send it.
what I meant is, can we just do not even send that ZERO flag at all? :)
>
> For the free page optimization, we skip free pages (could be thought of as 0
> pages in this context). The zero pages are detected at the time guest
> reports it QEMU. The page won't be reported if it is non-zero (i.e. used).
Sorry I must have not explained myself well. Let's assume the page
hint is used. I meant this:
- start precopy, page P is non-zero (let's say, page has content P1,
which is non-zero)
- we send page P with content P1 on src, then latest destination cache
of page P is P1
- page P is freed by the guest, then it becomes zero, dirty bitmap of
P is set since it's changed (from P1 to zeroed page)
- page P is provided as hint that we can skip it since it's zeroed,
then the dirty bit of P is cleared
- ... (page P is never used until migration completes)
After migration completes, page P should be an zeroed page on the
source, while IIUC on the destination side it's still with stale data
P1. Did I miss anything important?
Thanks,
--
Peter Xu