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[Qemu-devel] qemu and transparent huge pages
From: |
Michael Tokarev |
Subject: |
[Qemu-devel] qemu and transparent huge pages |
Date: |
Wed, 15 Aug 2012 16:45:04 +0400 |
User-agent: |
Mozilla/5.0 (X11; Linux i686 on x86_64; rv:10.0.5) Gecko/20120624 Icedove/10.0.5 |
[Reposting with the right email address of Andrea]
Quite some time ago there was a thread on qemu-devel,
started by Andrea, about modifying qemu to better
use transparent huge pages:
http://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01250.html
That thread hasn't reached any conclusion, but some time
after that Avi implemented a similar change:
commit 36b586284e678da28df3af9fd0907d2b16f9311c
Author: Avi Kivity <address@hidden>
Date: Mon Sep 5 11:07:05 2011 +0300
qemu_vmalloc: align properly for transparent hugepages and KVM
To make good use of transparent hugepages, KVM requires that guest-physical
and host-virtual addresses share the low 21 bits (as opposed to just the low
12 bits normally required).
Adjust qemu_vmalloc() to honor that requirement. Ignore it for small region
to avoid fragmentation.
Signed-off-by: Avi Kivity <address@hidden>
Signed-off-by: Anthony Liguori <address@hidden>
diff --git a/oslib-posix.c b/oslib-posix.c
index 196099c..a304fb0 100644
--- a/oslib-posix.c
+++ b/oslib-posix.c
@@ -35,6 +35,13 @@
extern int daemon(int, int);
#endif
+#if defined(__linux__) && defined(__x86_64__)
+ /* Use 2MB alignment so transparent hugepages can be used by KVM */
+# define QEMU_VMALLOC_ALIGN (512 * 4096)
+#else
+# define QEMU_VMALLOC_ALIGN getpagesize()
+#endif
+
#include "config-host.h"
#include "sysemu.h"
#include "trace.h"
@@ -80,7 +87,12 @@ void *qemu_memalign(size_t alignment, size_t size)
void *qemu_vmalloc(size_t size)
{
void *ptr;
- ptr = qemu_memalign(getpagesize(), size);
+ size_t align = QEMU_VMALLOC_ALIGN;
+
+ if (size < align) {
+ align = getpagesize();
+ }
+ ptr = qemu_memalign(align, size);
trace_qemu_vmalloc(size, ptr);
return ptr;
}
(why it is 64bit-only is a different, unrelated question).
But apparently, THP does not work still, even with 2Mb
alignment: when running a guest, AnonHugePages in
/proc/meminfo stays at 0 - either in kvm mode or in tcg
mode. Any idea why? What else is needed for THP to work?
This is quite a frequent question in #kvm IRC channel,
and I always suggested using -mem-path for this, but
I'm curios why it doesn't work automatically when it
probably should?
Thanks,
/mjt