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SubjectRe: Intel P6 vs P7 system call performance

On Mon, 16 Dec 2002, Linus Torvalds wrote:
>
> (Modulo the missing syscall page I already mentioned and potential bugs
> in the code itself, of course ;)

Ok, I did the vsyscall page too, and tried to make it do the right thing
(but I didn't bother to test it on a non-SEP machine).

I'm pushing the changes out right now, but basically it boils down to the
fact that with these changes, user space can instead of doing an

int $0x80

instruction for a system call just do a

call 0xfffff000

instead. The vsyscall page will be set up to use sysenter if the CPU
supports it, and if it doesn't, it will just do the old "int $0x80"
instead (and it could use the AMD syscall instruction if it wants to).
User mode shouldn't know or care, the calling convention is the same as it
ever was.

On my P4 machine, a "getppid()" is 641 cycles with sysenter/sysexit, and
something like 1761 cycles with the old "int 0x80/iret" approach. That's a
noticeable improvement, but I have to say that I'm a bit disappointed in
the P4 still, it shouldn't be even that much.

As a comparison, an Athlon will do a full int/iret faster than a P4 does a
sysenter/sysexit. Pathetic. But it's better than it used to be.

Whatever. The code is extremely simple, and while I'm sure there are
things I've missed I'd love to hear if this works for anybody else. I'm
appending the (extremely stupid) test-program I used to test it.

The way I did this, things like system call restarting etc _should_ all
work fine even with "sysenter", simply by virtue of both sysenter and "int
0x80" being two-byte opcodes. But it might be interesting to verify that a
recompiled glibc (or even just a preload) really works with this on a
"whole system" testbed rather than just testing one system call (and not
even caring about the return value) a million times.

The good news is that the kernel part really looks pretty clean.

Linus

---
#include <time.h>
#include <sys/time.h>
#include <asm/unistd.h>
#include <sys/stat.h>
#include <stdio.h>

#define rdtsc() ({ unsigned long a,d; asm volatile("rdtsc":"=a" (a), "=d" (d)); a; })

int main()
{
int i, ret;
unsigned long start, end;

start = rdtsc();
for (i = 0; i < 1000000; i++) {
asm volatile("call 0xfffff000"
:"=a" (ret)
:"0" (__NR_getppid));
}
end = rdtsc();
printf("%f cycles\n", (end - start) / 1000000.0);

start = rdtsc();
for (i = 0; i < 1000000; i++) {
asm volatile("int $0x80"
:"=a" (ret)
:"0" (__NR_getppid));
}
end = rdtsc();
printf("%f cycles\n", (end - start) / 1000000.0);
}


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