Re: "tell us why you want to disable CONFIG_COMPACTION" --> spikes from VMs?

[Matthew Wilcox <willy@infradead.org>]

On Sat, Dec 19, 2020 at 07:48:58PM +0100, Jason A. Donenfeld wrote:
> Hi,
> 
> In trying to debug an issue, I came across this text in the config
> COMPACTION menu item:
> 
> > Compaction is the only memory management component to form
> > high order (larger physically contiguous) memory blocks
> > reliably. The page allocator relies on compaction heavily and
> > the lack of the feature can lead to unexpected OOM killer
> > invocations for high order memory requests. You shouldn't
> > disable this option unless there really is a strong reason for
> > it and then we would be really interested to hear about that at
> > linux-mm@kvack.org.
> 
> So, here I am as requested.
> 
> My system has 64 gigs of ram. Often times when starting up VMs, with
> either QEMU/KVM or with VMware, the VM will be extremely sluggish and
> laggy for a little while, usually around 5 minutes. During that phase,
> I see kcompact0 using 100% CPU. But other times this doesn't happen at
> all. I figured I'd try removing CONFIG_COMPACTION and seeing what
> happens, but then I saw your admonishment, so here I am.

My initial question is "What is kcompactd doing?"  There are a number
of tracepoints in kcompactd, so you should be able to enable them all
and find out.