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SubjectRe: [PATCH] Re: Negative scalability by removal of
Linus Torvalds wrote:
>
> On Tue, 7 Nov 2000, Andrew Morton wrote:
>
> > Alan Cox wrote:
> > >
> > > > Even 2.2.x can be fixed to do the wake-one for accept(), if required.
> > >
> > > Do we really want to retrofit wake_one to 2.2. I know Im not terribly keen to
> > > try and backport all the mechanism. I think for 2.2 using the semaphore is a
> > > good approach. Its a hack to fix an old OS kernel. For 2.4 its not needed
> >
> > It's a 16-liner! I'll cheerfully admit that this patch
> > may be completely broken, but hey, it's free. I suggest
> > that _something_ has to be done for 2.2 now, because
> > Apache has switched to unserialised accept().
>
> This is why I'd love to _not_ see silly work-arounds in apache: we
> obviously _can_ fix the places where our performance sucks, but only if we
> don't have other band-aids hiding the true issues.
>
> For example, with a file-locking apache, we'd have to fix the (noticeably
> harder) file locking thing to be wake-one instead, and even then we'd
> never be able to do as well as something that gets the same wake-one thing
> without the two extra system calls.
>
> The patch looks superficially fine to me, although it does seem to add
> another cache-line to the wakeup setup - it migth be worth-while to have
> the exclusive state closer. But maybe I just didn't count right.

Your counting's fine. But I figured the third cachline was OK
because we're going to need that in add_to_runqueue() a few
cycles later.

Anyway, version 2 below uses LIFO for the accept() wakeups. This
appears to be a 5%-10% win for Apache. The browsing loop for
exclusive tasks will now pull in cachelines 0 and 2, rather
than the previous 0 and 1.

--- linux-2.2.18-pre19/include/linux/sched.h Sun Nov 5 11:46:54 2000
+++ linux-akpm/include/linux/sched.h Tue Nov 7 20:20:13 2000
@@ -79,6 +79,7 @@
#define TASK_ZOMBIE 4
#define TASK_STOPPED 8
#define TASK_SWAPPING 16
+#define TASK_EXCLUSIVE 32

/*
* Scheduling policies
@@ -251,6 +252,7 @@
struct task_struct *next_task, *prev_task;
struct task_struct *next_run, *prev_run;

+ unsigned int task_exclusive; /* task wants wake-one semantics in __wake_up() */
/* task state */
struct linux_binfmt *binfmt;
int exit_code, exit_signal;
@@ -370,6 +372,7 @@
/* counter */ DEF_PRIORITY,DEF_PRIORITY,0, \
/* SMP */ 0,0,0,-1, \
/* schedlink */ &init_task,&init_task, &init_task, &init_task, \
+/* task_exclusive */ 0, \
/* binfmt */ NULL, \
/* ec,brk... */ 0,0,0,0,0,0, \
/* pid etc.. */ 0,0,0,0,0, \
@@ -496,8 +499,8 @@
signed long timeout));
extern void FASTCALL(wake_up_process(struct task_struct * tsk));

-#define wake_up(x) __wake_up((x),TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE)
-#define wake_up_interruptible(x) __wake_up((x),TASK_INTERRUPTIBLE)
+#define wake_up(x) __wake_up((x),TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE | TASK_EXCLUSIVE)
+#define wake_up_interruptible(x) __wake_up((x),TASK_INTERRUPTIBLE | TASK_EXCLUSIVE)

#define __set_current_state(state_value) do { current->state = state_value; } while (0)
#ifdef __SMP__
--- linux-2.2.18-pre19/kernel/sched.c Sun Nov 5 11:46:54 2000
+++ linux-akpm/kernel/sched.c Tue Nov 7 20:23:25 2000
@@ -890,8 +890,9 @@
*/
void __wake_up(struct wait_queue **q, unsigned int mode)
{
- struct task_struct *p;
+ struct task_struct *p, *last_exclusive;
struct wait_queue *head, *next;
+ unsigned int done_exclusive, do_exclusive;

if (!q)
goto out;
@@ -906,10 +907,17 @@
if (!next)
goto out_unlock;

+ last_exclusive = NULL;
+ do_exclusive = mode & TASK_EXCLUSIVE;
while (next != head) {
p = next->task;
next = next->next;
if (p->state & mode) {
+ if (do_exclusive && p->task_exclusive) {
+ last_exclusive = p;
+ continue;
+ }
+
/*
* We can drop the read-lock early if this
* is the only/last process.
@@ -922,6 +930,8 @@
wake_up_process(p);
}
}
+ if (last_exclusive)
+ wake_up_process(last_exclusive);
out_unlock:
read_unlock(&waitqueue_lock);
out:
--- linux-2.2.18-pre19/net/ipv4/tcp.c Sun Nov 5 11:46:54 2000
+++ linux-akpm/net/ipv4/tcp.c Tue Nov 7 20:20:13 2000
@@ -1619,6 +1619,7 @@
struct wait_queue wait = { current, NULL };
struct open_request *req;

+ current->task_exclusive = 1;
add_wait_queue(sk->sleep, &wait);
for (;;) {
current->state = TASK_INTERRUPTIBLE;
@@ -1632,6 +1633,8 @@
break;
}
current->state = TASK_RUNNING;
+ wmb();
+ current->task_exclusive = 0;
remove_wait_queue(sk->sleep, &wait);
return req;
}
-
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