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[Qemu-devel] Performance regression using KVM/ARM

From: Christoffer Dall
Subject: [Qemu-devel] Performance regression using KVM/ARM
Date: Thu, 21 Apr 2016 18:23:48 +0200
User-agent: Mutt/1.5.21 (2010-09-15)


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!


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