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do_select() improvement ...

DISCUSSION Davide Libenzi 1 participant 2 messages Jun 21, 2001

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Davide Libenzi

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#0 lore raw

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

*** 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) {

author
#1 lore raw
Quoted 7 lines · 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.

Quoted 7 lines
                        off = i / __NFDBITS;

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

This is right.

- Davide

select.c renamed from select.orig.c +9 -1
@@ -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;