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

From: Christoffer Dall
Subject: Re: [Qemu-devel] Performance regression using KVM/ARM
Date: Fri, 22 Apr 2016 12:15:46 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Apr 22, 2016 at 12:06:52PM +0200, Alexander Graf wrote:
> On 04/22/2016 12:01 PM, Christoffer Dall wrote:
> >On Thu, Apr 21, 2016 at 09:50:05PM +0200, Alexander Graf wrote:
> >>
> >>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?
> >>
> >>
> >Unfortunately this doesn't make any difference.  It feels to me like
> >we're missing specifying a 2M alignemnt in QEMU somewhere, but I can't
> >properly understand the links between the actual allocation, registering
> >mem slots with the KVM part of QEMU, and actually setting up KVM user
> >memory regions.
> >
> >What has to happen is that the resulting struct
> >kvm_userspace_memory_region() has the same alignment offset from 2M (the
> >huge page size) of the ->guest_phys_addr and ->userspace-addr fields.
> Well, I would expect that the guest address space is always very big
> aligned - and definitely at least 2MB - so we're safe there.
> That means we only need to align the qemu virtual address. There
> used to be a memalign() call for that, but it got replaced with
> direct mmap() and then a lot of code changed on top. Looking at the
> logs, I'm sure Paolo knows the answer though :)
Peter just pointed me to a change I remember doing for ARM, so perhaps
this fix is the right one?

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index d25f671..a36e734 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -35,7 +35,7 @@
 extern int daemon(int, int);
-#if defined(__linux__) && (defined(__x86_64__) || defined(__arm__))
+#if defined(__linux__) && (defined(__x86_64__) || defined(__arm__)) || 
    /* Use 2 MiB alignment so transparent hugepages can be used by KVM.
       Valgrind does not support alignments larger than 1 MiB,
       therefore we need special code which handles running on Valgrind. */


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