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SubjectRe: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
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On Thu, 20 May 2010 02:31:50 pm Rusty Russell wrote:
> On Wed, 19 May 2010 05:36:42 pm Avi Kivity wrote:
> > > Note that this is a exclusive->shared->exclusive bounce only, too.
> > >
> >
> > A bounce is a bounce.
>
> I tried to measure this to show that you were wrong, but I was only able
> to show that you're right. How annoying. Test code below.

This time for sure!

#define _GNU_SOURCE
#include <unistd.h>
#include <sched.h>
#include <err.h>
#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/time.h>
#include <sys/mman.h>

/* We share memory via an mmap. */
struct counter {
unsigned int cacheline1;
char pad[256];
unsigned int cacheline2;
};
#define MAX_BOUNCES 100000000
enum mode {
SHARE,
UNSHARE,
LOCKSHARE,
LOCKUNSHARE,
};
int main(int argc, char *argv[])
{
cpu_set_t cpuset;
volatile struct counter *counter;
struct timeval start, stop;
bool child;
unsigned int count;
uint64_t usec;
enum mode mode;
if (argc != 4)
errx(1, "Usage: cachebounce share|unshare|lockshare|lockunshare <cpu0> <cpu1>");
if (strcmp(argv[1], "share") == 0)
mode = SHARE;
else if (strcmp(argv[1], "unshare") == 0)
mode = UNSHARE;
else if (strcmp(argv[1], "lockshare") == 0)
mode = LOCKSHARE;
else if (strcmp(argv[1], "lockunshare") == 0)
mode = LOCKSHARE;
else
errx(1, "Usage: cachebounce share|unshare|lockshare|lockunshare <cpu0> <cpu1>");
CPU_ZERO(&cpuset);

counter = mmap(NULL, getpagesize(), PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_SHARED, -1, 0);
if (counter == MAP_FAILED)
err(1, "Mapping page");
/* Fault it in. */
counter->cacheline1 = counter->cacheline2 = 0;
child = (fork() == 0);

CPU_SET(atoi(argv[2 + child]), &cpuset);
if (sched_setaffinity(getpid(), sizeof(cpu_set_t), &cpuset) != 0)
err(1, "Calling sched_setaffinity()");
gettimeofday(&start, NULL);

if (child) {
count = 1;
switch (mode) {
case SHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (counter->cacheline1 != count);
count++;
counter->cacheline1 = count;
count++;
}
break;
case UNSHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (counter->cacheline1 != count);
count++;
counter->cacheline2 = count;
count++;
}
break;
case LOCKSHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (__sync_val_compare_and_swap(&counter->cacheline1, count, count+1)
!= count);
count += 2;
}
break;
case LOCKUNSHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (counter->cacheline1 != count);
__sync_val_compare_and_swap(&counter->cacheline2, count, count+1);
count += 2;
}
break;
}
} else {
count = 0;
switch (mode) {
case SHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (counter->cacheline1 != count);
count++;
counter->cacheline1 = count;
count++;
}
break;
case UNSHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (counter->cacheline2 != count);
count++;
counter->cacheline1 = count;
count++;
}
break;
case LOCKSHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (__sync_val_compare_and_swap(&counter->cacheline1, count, count+1)
!= count);
count += 2;
}
break;
case LOCKUNSHARE:
while (count < MAX_BOUNCES) {
/* Spin waiting for other side to change it. */
while (counter->cacheline2 != count);
__sync_val_compare_and_swap(&counter->cacheline1, count, count+1);
count += 2;
}
break;
}
}
gettimeofday(&stop, NULL);
usec = (stop.tv_sec * 1000000LL + stop.tv_usec)
- (start.tv_sec * 1000000LL + start.tv_usec);
printf("CPU %s: %s cacheline: %llu usec\n", argv[2+child], argv[1], usec);
return 0;
}


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