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

From: Alexander Graf
Subject: Re: [Qemu-devel] Performance regression using KVM/ARM
Date: Thu, 21 Apr 2016 21:50:05 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:38.0) Gecko/20100101 Thunderbird/38.7.2

On 21.04.16 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!

The code changed quite heavily since I last looked at it, but could you
please try whether the (untested) patch below makes a difference?


diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index 0b4cc7f..24e73b1 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -36,7 +36,7 @@ size_t qemu_fd_getpagesize(int fd)

-    return getpagesize();
+    return 2 * 1024 * 1024;

 void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)

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