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SubjectRe: [PATCH] [RFC] Optimize select/poll by putting small data sets on the stack
Eric Dumazet a écrit :
> Andi Kleen a écrit :
>> This is a RFC for now. I would be interested in testing
>> feedback. Patch is for 2.6.15.
>>
>> Optimize select and poll by a using stack space for small fd sets
>>
>> This brings back an old optimization from Linux 2.0. Using
>> the stack is faster than kmalloc. On a Intel P4 system
>> it speeds up a select of a single pty fd by about 13%
>> (~4000 cycles -> ~3500)
>
> Was this result on UP or SMP kernel ? Preempt or not ?
>
> I think we might play in do_pollfd() and use fget_light()/fput_light()
> instead of fget()/fput() that are somewhat expensive because of atomic
> inc/dec on SMP.
>

Just for completeness I include this patch against 2.6.15


--- linux-2.6.15/fs/select.c 2006-01-03 04:21:10.000000000 +0100
+++ linux-2.6.15-ed/fs/select.c 2006-01-04 00:04:10.000000000 +0100
@@ -221,17 +221,18 @@
}

for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) {
+ int fput_needed;
if (i >= n)
break;
if (!(bit & all_bits))
continue;
- file = fget(i);
+ file = fget_light(i, &fput_needed);
if (file) {
f_op = file->f_op;
mask = DEFAULT_POLLMASK;
if (f_op && f_op->poll)
mask = (*f_op->poll)(file, retval ? NULL : wait);
- fput(file);
+ fput_light(file, fput_needed);
if ((mask & POLLIN_SET) && (in & bit)) {
res_in |= bit;
retval++;
@@ -417,14 +418,15 @@
fdp = fdpage+i;
fd = fdp->fd;
if (fd >= 0) {
- struct file * file = fget(fd);
+ int fput_needed;
+ struct file * file = fget_light(fd, &fput_needed);
mask = POLLNVAL;
if (file != NULL) {
mask = DEFAULT_POLLMASK;
if (file->f_op && file->f_op->poll)
mask = file->f_op->poll(file, *pwait);
mask &= fdp->events | POLLERR | POLLHUP;
- fput(file);
+ fput_light(file, fput_needed);
}
if (mask) {
*pwait = NULL;
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