linux-kernel - Re: [RFC PATCH 0/4] KVM: ioctl for populating guest

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Message-ID: <03a12598-74aa-4202-a79a-668b45dbcc47@amazon.com>
Date: Wed, 20 Nov 2024 17:21:36 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
To: David Hildenbrand <david@...hat.com>, <pbonzini@...hat.com>,
	<corbet@....net>, <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
CC: <jthoughton@...gle.com>, <brijesh.singh@....com>, <michael.roth@....com>,
	<graf@...zon.de>, <jgowans@...zon.com>, <roypat@...zon.co.uk>,
	<derekmn@...zon.com>, <nsaenz@...zon.es>, <xmarcalx@...zon.com>, "Sean
 Christopherson" <seanjc@...gle.com>, <linux-mm@...ck.org>
Subject: Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd



On 20/11/2024 16:44, David Hildenbrand wrote:
>> If the problem is the "pagecache" overhead, then yes, it will be a
>> harder nut to crack. But maybe there are some low-hanging fruits to
>> optimize? Finding the main cause for the added overhead would be
>> interesting.

Agreed, knowing the exact root cause would be really nice.

> Can you compare uffdio_copy() when using anonymous memory vs. shmem?
> That's likely the best we could currently achieve with guest_memfd.

Yeah, I was doing that too. It was about ~28% slower in my setup, while 
with guest_memfd it was ~34% slower.  The variance of the data was quite 
high so the difference may well be just noise.  In other words, I'd be 
much happier if we could bring guest_memfd (or even shmem) performance 
closer to the anon/private than if we just equalised guest_memfd with 
shmem (which are probably already pretty close).

> There is the tools/testing/selftests/mm/uffd-stress benchmark, not sure
> if that is of any help; it SEGFAULTS for me right now with a (likely)
> division by 0.

Thanks for the pointer, will take a look!

> Cheers,
> 
> David / dhildenb
>