Hi all,
In many application we use select() system call for delay.
example:
select(0,NULL,NULL,NULL,&t1);
select() for delay is very inefficient. I modified sys_select() code for
efficiency .Here are the changes to fs/select.c.
Please suggest on these changes.
I know nanosleep() can be used instead of select(), but please suggest
on my changes.
file : fs/select.c
function : sys_select()
timeout += sec * (unsigned long) HZ;
}
}
-
+
+
ret = -EINVAL;
if (n < 0)
goto out_nofds;
-
+ if ( (n == 0) && (inp == NULL) && (outp == NULL) &&
(exp== NULL)){
+ printf("\n I am inside new select condition timeout
%d\n",timeout);
+ set_current_state(TASK_INTERRUPTIBLE);
+ ret = 0;
+ timeout = schedule_timeout(timeout);
+ if (signal_pending(current))
+ ret = -ERESTARTNOHAND;
+ if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
+ time_t sec = 0, usec = 0;
+ if (timeout) {
+ sec = timeout / HZ;
+ usec = timeout % HZ;
+ usec *= (1000000/HZ);
+ }
+ put_user(sec, &tvp->tv_sec);
+ put_user(usec, &tvp->tv_usec);
+ }
+ current->state = TASK_RUNNING;
+ goto out_nofds;
+ }
+
/* max_fdset can increase, so grab it once to avoid race */
max_fdset = current->files->max_fdset;
if (n > max_fdset)
Thank you very much.
Thanks for your assistances.
Madhu K.S.
On 10/6/05, Madhu K.S. <[email protected]> wrote:
> Hi all,
>
>
> In many application we use select() system call for delay.
>
> example:
> select(0,NULL,NULL,NULL,&t1);
>
>
> select() for delay is very inefficient. I modified sys_select() code for
> efficiency .Here are the changes to fs/select.c.
>
> Please suggest on these changes.
>
A few tiny comments below.
> I know nanosleep() can be used instead of select(), but please suggest
> on my changes.
>
>
> file : fs/select.c
> function : sys_select()
>
Submitting an actual applyable patch is preferred. Makes it possible
to easily apply your changes to test the changes and then the file and
function is also part of the patch so you won't have to spell that out
explicitly.
Use diff -up
>
>
>
> timeout += sec * (unsigned long) HZ;
> }
> }
> -
> +
> +
> ret = -EINVAL;
> if (n < 0)
> goto out_nofds;
> -
> + if ( (n == 0) && (inp == NULL) && (outp == NULL) &&
> (exp== NULL)){
No space for the beginning parenthesis and space before the opening
bracket is preferred:
if ((n == 0) && (inp == NULL) && (outp == NULL) &&
(exp== NULL)) {
> + printf("\n I am inside new select condition timeout
> %d\n",timeout);
Having a printk() here certainly won't help performance.
> + set_current_state(TASK_INTERRUPTIBLE);
> + ret = 0;
> + timeout = schedule_timeout(timeout);
> + if (signal_pending(current))
> + ret = -ERESTARTNOHAND;
Wouldn't it make sense to jump out at this point if there's a signal pending?
if (signal_pending(current)) {
ret = -ERESTARTNOHAND;
goto out;
}
Or am I missing something?
> + if (tvp && !(current->personality & STICKY_TIMEOUTS)) {
> + time_t sec = 0, usec = 0;
> + if (timeout) {
> + sec = timeout / HZ;
> + usec = timeout % HZ;
> + usec *= (1000000/HZ);
Small style thing: usec *= (1000000 / HZ);
> + }
> + put_user(sec, &tvp->tv_sec);
> + put_user(usec, &tvp->tv_usec);
> + }
> + current->state = TASK_RUNNING;
> + goto out_nofds;
> + }
> +
> /* max_fdset can increase, so grab it once to avoid race */
> max_fdset = current->files->max_fdset;
> if (n > max_fdset)
>
[snip]
--
Jesper Juhl <[email protected]>
Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please http://www.expita.com/nomime.html
On 10/6/05, Madhu K.S. <[email protected]> wrote:
> Hi all,
>
> In many application we use select() system call for delay.
>
> example:
> select(0,NULL,NULL,NULL,&t1);
>
> select() for delay is very inefficient. I modified sys_select() code for
Why don't you just use nanosleep(2) (or usleep)?
On 10/6/05, Jesper Juhl <[email protected]> wrote:
> On 10/6/05, Madhu K.S. <[email protected]> wrote:
[snip]
> > + usec *= (1000000/HZ);
> Small style thing: usec *= (1000000 / HZ);
>
Ohh and the parenthesis are not needed.
usec *= 1000000 / HZ;
[snip]
--
Jesper Juhl <[email protected]>
Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please http://www.expita.com/nomime.html
Alex Riesen wrote:
> Why don't you just use nanosleep(2) (or usleep)?
I can think of one main reason...existing code. Also, nanosleep()
rounds up excessively in many kernel versions, so that a request to
sleep for less than 1 tick ends up sleeping for 2 ticks.
The select() man page explicitly mentions this usage;
"Some code calls select with all three sets empty, n zero, and a
non-null timeout as a fairly portable way to sleep with subsecond
precision."
Chris
On Thu, 2005-10-06 at 09:26 -0600, Christopher Friesen wrote:
> Alex Riesen wrote:
>
> > Why don't you just use nanosleep(2) (or usleep)?
>
> I can think of one main reason...existing code. Also, nanosleep()
And it's cooler to hack the kernel than to create and use a
portable_sleep() function and use it.
> rounds up excessively in many kernel versions, so that a request to
> sleep for less than 1 tick ends up sleeping for 2 ticks.
^^^^^^^
> The select() man page explicitly mentions this usage;
>
> "Some code calls select with all three sets empty, n zero, and a
> non-null timeout as a fairly portable way to sleep with subsecond
^^^^^^^^^
> precision."
^^^^^^^^^
You do realize that "subsecond precision" is probably meant as
improvement to sleep(3) and surely not to nanosleep(2)?
Bernd
--
Firmix Software GmbH http://www.firmix.at/
mobil: +43 664 4416156 fax: +43 1 7890849-55
Embedded Linux Development and Services
Bernd Petrovitsch wrote:
> On Thu, 2005-10-06 at 09:26 -0600, Christopher Friesen wrote:
> And it's cooler to hack the kernel than to create and use a
> portable_sleep() function and use it.
If there is a substantial codebase using select() for sleeping, then it
makes sense to improve the efficiency of the kernel. Fix it in one
place, make all the apps run better.
>>The select() man page explicitly mentions this usage;
>>
>>"Some code calls select with all three sets empty, n zero, and a
>>non-null timeout as a fairly portable way to sleep with subsecond
> ^^^^^^^^^
>>precision."
> ^^^^^^^^^
>
> You do realize that "subsecond precision" is probably meant as
> improvement to sleep(3) and surely not to nanosleep(2)?
select() allows for the selection of sleep time with microsecond
precision. The mainline kernel can't sleep for that small an interval
anyway, so there's not really any difference in sleep precision between
the two.
As I mentioned earlier, select() actually sleeps more accurately than
nanosleep() on many kernels. I haven't tested the most recent to see if
this is still true though.
Chris
> The select() man page explicitly mentions this usage;
>
> "Some code calls select with all three sets empty, n zero, and a
> non-null timeout as a fairly portable way to sleep with subsecond
> precision."
Perl's documentation also notes it as one of the many ways to do a
subsecond sleep:
>You can effect a sleep of 250 milliseconds this way:
>
> select(undef, undef, undef, 0.25);
TMTOWTDI.
Jesper Juhl wrote:
> Or am I missing something?
>
Insert obligatory joke about optimizing delay loops... ?
--
-- Howard Chu
Chief Architect, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc
OpenLDAP Core Team http://www.openldap.org/project/
Howard Chu wrote:
> Insert obligatory joke about optimizing delay loops... ?
It's not a delay if there's a signal pending.
Chris
Alex Riesen wrote:
> On 10/6/05, Madhu K.S. <[email protected]> wrote:
>
>>Hi all,
>>
>>In many application we use select() system call for delay.
>>
>>example:
>>select(0,NULL,NULL,NULL,&t1);
>>
>>select() for delay is very inefficient. I modified sys_select() code for
>
>
> Why don't you just use nanosleep(2) (or usleep)?
I think the answers here are (a) because there's an existing code base,
and (b) as long as the functionality is required it might as well be
provided optimally. If tou're going to do something at all, do it right.
--
-bill davidsen ([email protected])
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me