From: Christoph Hellwig <hch@infradead.org>
Originally by David Mosberger, testing by Roger Luethi. From the ia32 tree.
Basically, it avoids going to memory all the time. What this does is make
life a lot easier for gcc, so it can actually do a decent amount of
optimization. The restructuring clearly is less important for out-of-order
CPUs, but even there it gives some benefits.
More specifically, the loop is now structured to operate one "unsigned long"
at a time, rather than one bit at a time. Of course, you still need to
process all the bits, but most of the relevant state in the inner loop can be
kept in registers.
Roger Luethi measured the routine on a bunch of different machines (mostly
x86, IIRC: P5, P6, Crusoe, Athlons) and performance improved there, too (and
it should definitely improve performance on any RISC-like architecture).
Roger's benchmarking results (vs number of fd's):
File TCP
Numbfer of fd's: 10 250 500 10 250 500
UP, Pentium MMX 233MHz original 8.2 108.5 212.8 11.0 180.0 356.5
UP, Pentium MMX 233MHz w/patch 7.4 87.6 171.1 10.4 163.6 323.4
MP, Pentium MMX 233MHz original 15.7 283.8 562.8 18.9 354.4 705.5
MP, Pentium MMX 233MHz w/patch 14.6 255.6 506.5 17.8 332.8 664.1
UP, Athlon 1394 MHz original 1.3 13.4 26.1 1.9 24.7 48.6
UP, Athlon 1394 MHz w/patch 1.2 11.0 21.5 1.6 22.3 43.8
MP, Athlon 1394 MHz original 1.6 22.4 44.6 1.9 30.9 60.5
MP, Athlon 1394 MHz w/patch 1.5 21.2 41.7 1.9 30.2 59.6
25-akpm/fs/select.c | 75 ++++++++++++++++++++++++++++++++--------------------
1 files changed, 47 insertions(+), 28 deletions(-)
diff -puN fs/select.c~select-speedup fs/select.c
--- 25/fs/select.c~select-speedup Thu Apr 17 16:53:41 2003
+++ 25-akpm/fs/select.c Thu Apr 17 16:53:41 2003
@@ -176,7 +176,7 @@ int do_select(int n, fd_set_bits *fds, l
{
struct poll_wqueues table;
poll_table *wait;
- int retval, i, off;
+ int retval, i;
long __timeout = *timeout;
spin_lock(¤t->files->file_lock);
@@ -193,38 +193,57 @@ int do_select(int n, fd_set_bits *fds, l
wait = NULL;
retval = 0;
for (;;) {
+ unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;
+
set_current_state(TASK_INTERRUPTIBLE);
- for (i = 0 ; i < n; i++) {
- unsigned long bit = BIT(i);
- unsigned long mask;
- struct file *file;
- off = i / __NFDBITS;
- if (!(bit & BITS(fds, off)))
+ inp = fds->in; outp = fds->out; exp = fds->ex;
+ rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;
+
+ for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
+ unsigned long in, out, ex, all_bits, bit = 1, mask, j;
+ unsigned long res_in = 0, res_out = 0, res_ex = 0;
+ struct file_operations *f_op = NULL;
+ struct file *file = NULL;
+
+ in = *inp++; out = *outp++; ex = *exp++;
+ all_bits = in | out | ex;
+ if (all_bits == 0)
continue;
- file = fget(i);
- mask = POLLNVAL;
- if (file) {
+
+ for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) {
+ if (i >= n)
+ break;
+ if (!(bit & all_bits))
+ continue;
+ file = fget(i);
+ if (file)
+ f_op = file->f_op;
mask = DEFAULT_POLLMASK;
- if (file->f_op && file->f_op->poll)
- mask = file->f_op->poll(file, wait);
- fput(file);
- }
- if ((mask & POLLIN_SET) && ISSET(bit, __IN(fds,off))) {
- SET(bit, __RES_IN(fds,off));
- retval++;
- wait = NULL;
- }
- if ((mask & POLLOUT_SET) && ISSET(bit, __OUT(fds,off))) {
- SET(bit, __RES_OUT(fds,off));
- retval++;
- wait = NULL;
- }
- if ((mask & POLLEX_SET) && ISSET(bit, __EX(fds,off))) {
- SET(bit, __RES_EX(fds,off));
- retval++;
- wait = NULL;
+ if (file) {
+ if (f_op && f_op->poll)
+ mask = (*f_op->poll)(file, retval ? NULL : wait);
+ fput(file);
+ if ((mask & POLLIN_SET) && (in & bit)) {
+ res_in |= bit;
+ retval++;
+ }
+ if ((mask & POLLOUT_SET) && (out & bit)) {
+ res_out |= bit;
+ retval++;
+ }
+ if ((mask & POLLEX_SET) && (ex & bit)) {
+ res_ex |= bit;
+ retval++;
+ }
+ }
}
+ if (res_in)
+ *rinp = res_in;
+ if (res_out)
+ *routp = res_out;
+ if (res_ex)
+ *rexp = res_ex;
}
wait = NULL;
if (retval || !__timeout || signal_pending(current))
_