do_select() improvement ...

do_select() improvement ...

[Davide Libenzi]

[-- Attachment #1: Type: text/plain, Size: 2085 bytes --]


This patch can improve do_select() ==> select() performance due :

1) the load from the fd bitmap is done every __NFDBITS instead of every bit.
        If You look at the BITS() macro You'll see that it's not cheap in terms
        of memory operations

2) if a sequential hole of __NFDBITS is found it'll be skipped without doing a
        bit by bit check




*** select.orig.c       Thu Jun 21 08:52:04 2001
--- select.c    Thu Jun 21 12:09:25 2001
***************
*** 164,171 ****
--- 164,172 ----
  {
        poll_table table, *wait;
        int retval, i, off;
        long __timeout = *timeout;
+       unsigned long bits;
 
        read_lock(&current->files->file_lock);
        retval = max_select_fd(n, fds);
        read_unlock(&current->files->file_lock);
***************
*** 186,194 ****
                        unsigned long mask;
                        struct file *file;
 
                        off = i / __NFDBITS;
!                       if (!(bit & BITS(fds, off)))
                                continue;
                        file = fget(i);
                        mask = POLLNVAL;
                        if (file) {
--- 187,202 ----
                        unsigned long mask;
                        struct file *file;
 
                        off = i / __NFDBITS;
!                       if (!(i & (__NFDBITS - 1))) {
!                               bits = BITS(fds, off);
!                               if (!bits) {
!                                       i += __NFDBITS;
!                                       continue;
!                               }
!                       }
!                       if (!(bit & bits))
                                continue;
                        file = fget(i);
                        mask = POLLNVAL;
                        if (file) {
 
                                                                               
                                             

                                                                             
                                             

- Davide


[-- Attachment #2: select.c.diff --]
[-- Type: application/octet-stream, Size: 918 bytes --]

*** select.orig.c	Thu Jun 21 08:52:04 2001
--- select.c	Thu Jun 21 12:09:25 2001
***************
*** 164,171 ****
--- 164,172 ----
  {
  	poll_table table, *wait;
  	int retval, i, off;
  	long __timeout = *timeout;
+ 	unsigned long bits;
  
   	read_lock(&current->files->file_lock);
  	retval = max_select_fd(n, fds);
  	read_unlock(&current->files->file_lock);
***************
*** 186,194 ****
  			unsigned long mask;
  			struct file *file;
  
  			off = i / __NFDBITS;
! 			if (!(bit & BITS(fds, off)))
  				continue;
  			file = fget(i);
  			mask = POLLNVAL;
  			if (file) {
--- 187,202 ----
  			unsigned long mask;
  			struct file *file;
  
  			off = i / __NFDBITS;
! 			if (!(i & (__NFDBITS - 1))) {
! 				bits = BITS(fds, off);
! 				if (!bits) {
! 					i += __NFDBITS;
! 					continue;
! 				}
! 			}
! 			if (!(bit & bits))
  				continue;
  			file = fget(i);
  			mask = POLLNVAL;
  			if (file) {

RE: do_select() improvement ...

[Davide Libenzi]

[-- Attachment #1: Type: text/plain, Size: 799 bytes --]


On 21-Jun-2001 Davide Libenzi wrote:
>                         off = i / __NFDBITS;
> !                       if (!(i & (__NFDBITS - 1))) {
> !                               bits = BITS(fds, off);
> !                               if (!bits) {
> !                                       i += __NFDBITS;
> !                                       continue;
> !                               }


This is wrong.



>                         off = i / __NFDBITS;
> !                       if (!(i & (__NFDBITS - 1))) {
> !                               bits = BITS(fds, off);
> !                               if (!bits) {
> !                                       i += __NFDBITS - 1;
> !                                       continue;
> !                               }

This is right.





- Davide


[-- Attachment #2: select.c.diff --]
[-- Type: application/octet-stream, Size: 597 bytes --]

--- select.orig.c	Thu Jun 21 08:52:04 2001
+++ select.c	Thu Jun 21 12:09:25 2001
@@ -165,6 +165,7 @@
 	poll_table table, *wait;
 	int retval, i, off;
 	long __timeout = *timeout;
+	unsigned long bits;
 
  	read_lock(&current->files->file_lock);
 	retval = max_select_fd(n, fds);
@@ -187,7 +188,14 @@
 			struct file *file;
 
 			off = i / __NFDBITS;
-			if (!(bit & BITS(fds, off)))
+			if (!(i & (__NFDBITS - 1))) {
+				bits = BITS(fds, off);
+				if (!bits) {
+					i += __NFDBITS - 1;
+					continue;
+				}
+			}
+			if (!(bit & bits))
 				continue;
 			file = fget(i);
 			mask = POLLNVAL;