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Re: [Qemu-devel] Performance regression using KVM/ARM

From: Laszlo Ersek
Subject: Re: [Qemu-devel] Performance regression using KVM/ARM
Date: Thu, 21 Apr 2016 23:58:07 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2

On 04/21/16 18:23, Christoffer Dall wrote:
> Hi,
> Commit 9fac18f (oslib: allocate PROT_NONE pages on top of RAM,
> 2015-09-10) had the unfortunate side effect that memory slots registered
> with KVM no longer contain a userspace address that is aligned to a 2M
> boundary, causing the use of THP to fail in the kernel.
> I fail to see where in the QEMU code we should be asking for a 2M
> alignment of our memory region.  Can someone help pointing me to the
> right place to fix this or suggest a patch?
> This causes a performance regssion of hackbench on KVM/ARM of about 62%
> compared to the workload running with THP.
> We have verified that this is indeed the cause of the failure by adding
> various prints to QEMU and the kernel, but unfortunatley my QEMU
> knowledge is not sufficient for me to fix it myself.
> Any help would be much appreciated!

Can you please test the attached series?

(Note that I'm only interested in solving this problem as a productive
distraction, so if the patches don't work, or require a lot of massaging
for merging, I'll just drop them (or, preferably, give them to someone


Attachment: 0001-util-mmap-alloc-take-alignment-in-qemu_ram_munmap.patch
Description: Text Data

Attachment: 0002-util-mmap-alloc-factor-out-size_with_guard_pages.patch
Description: Text Data

Attachment: 0003-util-mmap-alloc-preserve-size-alignment-with-guard-p.patch
Description: Text Data

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