Re: [External] Re: [PATCH 0/6] Introduce multifd zero page checking.

[Peter Xu]

On Wed, Feb 07, 2024 at 04:47:27PM -0800, Hao Xiang wrote:
> Sure I will drop "throughput" to avoid confusion. In my testing, 1
> multifd channel already makes the main thread spin at 100%. So the
> total-time is the same across 1/2/4 multifd channels as long as zero
> page is run on the main migration thread. Of course, this is based on
> the fact that the network is not the bottleneck. One interesting
> finding is that multifd 1 channel with multifd zero page has better
> performance than multifd 1 channel with main migration thread.

It's probably because the main thread has even more works to do than
"detecting zero page" alone.

When zero detection is done in main thread and when the guest is fully
idle, it'll consume a major portion of main thread cpu resource scanning
those pages already.  Consider all pages zero, multifd threads should be
fully idle, so n_channels may not matter here.

When 1 multifd thread created with zero-page offloading, zero page is fully
offloaded from main -> multifd thread even if only one.  It's kind of a
similar effect of forking the main thread into two threads, so the main
thread can be more efficient on other tasks (fetching/scanning dirty bits,
etc.).

Thanks,

-- 
Peter Xu