Li, Xin B wrote:
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of
Ulrich Drepper
Sent: Friday, August 03, 2007 2:24 AM
To: Nakajima, Jun
Cc: kvm-devel@lists.sourceforge.net
Subject: Re: [kvm-devel] [PATCH 2/2] Add MSR Bitmap support in VMX
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
Nakajima, Jun wrote:
Looking at the Linux context switch code, it can bang on MSR_FS_BASE
and
MSR_KERNEL_GS_BASE. A high context switch rate between
threads (which
use %gs or %fs) can show an improvement with this. Things
like kbuild
probably won't as they use single threaded processes.
Right.
In Linux there is no difference between single- and multi-threaded
processes when it comes to setting up segment registers. Every x86-64
process uses %fs for TLS and every x86 process uses %gs. There are
always variables accessed this way (errno, for instance).
I have to say, the benefit form _no_ VMExits on these VMExits are not so
obviously, because each context switch invloves a guest CR3 update, then
a page fault vmexit is triggered and a shadow handling is involved,
which is the major cost of guest context switch.
A small test program demonstrates the benefit. In thread mode, it takes
5 seconds without the patch and 3 seconds with the patch applied (200K
and ~0 exits/sec). In fork mode, it takes 10 seconds without the patch
and 7.2 seconds with the patch (420K and 280K exits/sec). Note that
with this test there are no page fault exits, only cr and msr exits
(with the threaded mode avoiding the cr exit).
So the patch is quite measurable on microbenchmarks -- in threaded mode
we're at native performance.
--
error compiling committee.c: too many arguments to function
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
static int p1[2], p2[2];
#define N 1000000
void *server(void *_)
{
char buf;
while (read(p1[0], &buf, 1) == 1)
write(p2[1], &buf, 1);
}
void client()
{
char buf;
int i;
for (i = 0; i < N; ++i) {
write(p1[1], &buf, 1);
read(p2[0], &buf, 1);
}
}
int main(int ac, char **av)
{
pthread_t thread;
pipe(p1);
pipe(p2);
if (ac < 2 || strcmp(av[1], "fork") != 0)
pthread_create(&thread, NULL, server, NULL);
else
if (fork() == 0) {
server(NULL);
return 0;
}
client();
return 0;
}
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