Linux-kernel mailing list archive 2000-23,: PATCH: swap_out mega (100+ times) speedboost

# PATCH: swap_out mega (100+ times) speedboost

John Fremlin (vii@penguinpowered.com)
04 Jun 2000 16:43:12 +0100

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The following patch improves responsiveness (by an immodestly
ridiculous ammount) when there are hundreds of small processes running
on a machine. Without it, it takes a few minutes for even a SysReq SAK
to get through, with it, a few seconds (AMD K6-2 64Mb)!!! I am
somewhat excited as this is my first kernel patch (I *can't resist*
posting it to the kernel list).

It is my thesis that
1) there is no point in finding the largest process (modified
by size_cnt) in vmscan.c:swap_out as this is unfair and a bad
heuristic; and
2) spending ages searching through the task list to find the
very biggest is a waste of time and severely impacts performance; and
3) the size_cnt heuristic should be done someother way.

My patch proves that searching through the task list for the largest
size_cnt severely impacts performance when there are many (100s of)
threads.

As I explained (on linux-mm):
> Perhaps, but you're traversing a linked list. That means that the
> task_struct entries will probably be widely dispersed, so that each
> one has to be fetched from main RAM, then you look at the mm_struct
> (another miss?). According to "Modern Compiler Implementation in ML"
> (Andrew W. Appel, Cambridge University Press, 1998) a secondary cache
> miss is typically 100-200 cycles. So if we say around 300-400 cycles
> per iteration of the loop (assuming that the needed data in the two
> structs are fetched completely for each miss penalty), everything
> taken together. I'd say that's quite slow, but I guess assembly
> programming skews your outlook considerably ;-)

The patch is against 2.4.0test1-ac7 with Rik's mmpatch version 3. It
stops trying to get the biggest size_cnt on the task list
(vmscan.c:swap_out), instead just picking the first possible one to
swap out. That is, size_cnt is being (ab)used as a boolean. IMHO, I
think it should be rethoughtout, so take this patch as a technology
demonstration ;-)

Normal system performance does not seem to be affected, but
responsiveness under heavy load is increased considerably. Reports of
performance slowdowns in any situations of course welcome!

Hope I didn't break anything :=)

--- linux-2.4.0t1a7m3/mm/vmscan.c Sat Jun 3 17:10:15 2000
+++ kernel-hacking/mm/vmscan.c Sun Jun 4 16:35:31 2000
@@ -361,23 +361,24 @@
/*
* We make one or two passes through the task list, indexed by
* assign = {0, 1}:
- * Pass 1: select the swappable task with maximal RSS that has
- * not yet been swapped out.
+ *
+ * Pass 1: select the first swappable task that has not yet
+ * been swapped out.
+ *
* Pass 2: re-assign rss swap_cnt values, then select as above.
*
* With this approach, there's no need to remember the last task
* swapped out. If the swap-out fails, we clear swap_cnt so the
* task won't be selected again until all others have been tried.
*
- * Think of swap_cnt as a "shadow rss" - it tells us which process
- * we want to page out (always try largest first).
- */
+ * Think of swap_cnt as a "shadow rss" - it tells us which
+ * process we want to page out (always try largest first). */
+
counter = (nr_threads << 2) >> (priority >> 2);
if (counter < 1)
counter = 1;
for (; counter >= 0; counter--) {
- unsigned long max_cnt = 0;
struct mm_struct *best = NULL;
int pid = 0;
int assign = 0;
@@ -391,13 +392,14 @@
if (mm->rss <= 0)
continue;
/* Refresh swap_cnt? */
- if (assign == 1)
+ best = mm;
+ pid = p->pid;
+
+ if (assign == 1){
mm->swap_cnt = mm->rss;
- if (mm->swap_cnt > max_cnt) {
- max_cnt = mm->swap_cnt;
- best = mm;
- pid = p->pid;
}
+ else
+ break;
}
read_unlock(&tasklist_lock);
if (!best) {

    --

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