select(0,NULL,NULL,NULL,&t1) used for delay

select(0,NULL,NULL,NULL,&t1) used for delay

[Madhu K.S.]

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.

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Jesper Juhl]

On 10/6/05, Madhu K.S. <madhu.subbaiah@wipro.com> 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 <jesper.juhl@gmail.com>
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

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Jesper Juhl]

On 10/6/05, Jesper Juhl <jesper.juhl@gmail.com> wrote:
> On 10/6/05, Madhu K.S. <madhu.subbaiah@wipro.com> wrote:
[snip]
> > +                                usec *= (1000000/HZ);
> Small style thing:   usec *= (1000000 / HZ);
>
Ohh and the parenthesis are not needed.
                                 usec *= 1000000 / HZ;

[snip]
--
Jesper Juhl <jesper.juhl@gmail.com>
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

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Howard Chu]

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/

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Christopher Friesen]

Howard Chu wrote:

> Insert obligatory joke about optimizing delay loops... ?

It's not a delay if there's a signal pending.

Chris

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Alex Riesen]

On 10/6/05, Madhu K.S. <madhu.subbaiah@wipro.com> 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)?

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Christopher Friesen]

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

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Bernd Petrovitsch]

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

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Christopher Friesen]

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

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Bob Copeland]

> 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.

Re: select(0,NULL,NULL,NULL,&t1) used for delay

[Bill Davidsen]

Alex Riesen wrote:
> On 10/6/05, Madhu K.S. <madhu.subbaiah@wipro.com> 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 (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
  last possible moment - but no longer"  -me