Re: [PATCH] 2.2.1{3,4,5} VM fix

[Andrea Arcangeli <andrea@suse.de>]

On Thu, 20 Jan 2000, Rik van Riel wrote:

>On Thu, 20 Jan 2000, Andrea Arcangeli wrote:
>
>> There's a limit of max swap request, after that a wait_on_page
>> will trigger I/O. So you'll only decrease performance by doing so
>> as far I can tell.
>
>Not really. We want to get the I/O done and we don't want
>to wait until the queue fills up.

The bank gives us 32 pages of credit. We don't need to get the I/O on
them. We have a credit that we can use to optimze the I/O.

>> > 			tsk->state = TASK_INTERRUPTIBLE;
>> >-			schedule_timeout(10*HZ);
>> >+			schedule_timeout(HZ);
>> 
>> I used 10 sec because if you run oom into kswapd it means you
>> can't hardly do anything within kswapd in the next 10 seconds.
>
>OOM is not the normal state the system is in. You really want

And infact such schedule_timeout(10*HZ) will happen _only_ when you are
OOM.

>kswapd to do some work in the background (allowing user programs
>to allocate their bits of memory without stalling).

This should be accomplished by a pre-wakeup once the high watermark
triggers. The HZ polling that is just doing is only for atomic
allocations. To parallelize memory freeing a polling with HZ frequency is
too slow.

>> If instead atomic allocations caused oom and kswapd failed then
>> you should give up for some time as well since you know all memory
>> is not freeable and so only a __free_pages will release memory.
>
>In the OOM case we'll have to kill a process. Stalling the
>system forever is not a solution.

The system is not stalled. atomic stuff that doesn't need to alloc memory
will continue to run. It will then complete and release memory later. Or
it will fail if it needs futher allocation to complete and it will release
memory for other atomic stuff that can now start. Allowing the TCP stack
to kill netscape is not a 2.2.x thing IMHO. BTW, you are not doing that in
your patch either.

>> > 	wake_up_interruptible(&kswapd_wait);
>> >-	if (gfp_mask & __GFP_WAIT)
>> >+	if ((gfp_mask & __GFP_WAIT) && (nr_free_pages < (freepages.low - 4)))
>> > 		retval = do_try_to_free_pages(gfp_mask);
>> 
>> -4 make no sense as ".low" could be as well set to 3 and
>> theorically the system should remains stable.
>
>It does make sense in reality. Please read the explanation
>I sent with the patch. I agree that somewhere halfway
>freepages.low and freepages.min would be better, but there
>is no need to complicate the calculation...

The calc is a linear scale 1/3 2/3 3/3. The difference between 512 and 508
is zero in RL.

>> And you are trying to wakeup kswapd instead of blocking. This will
>> make oom condition worse and it brekas my trashing mem watermark
>> heuristic. The heuristic is necessary to penalize the hog. It
>> should be a per-process thing btw.
>
>If kswapd can keep up, there's no need at all to slow down
>processes, not even hogs. If kswapd can't keep up then we'll

If you don't want to slow down not-hogs-processes, you must make sure that
who is allocating memory will block. If kswapd will be the only blocking
the system will trash because the hog will continue to run.

>start stalling processes (just 5 allocations further away,
>_and_ kswapd will have been started by that time and have had
>the time to do something).

That's not a matter of swapout but of shrink_mmap. See my previous point
about a pre-wakeup when the high watermark triggers. When swap is involved
instead kswapd won't help anymore IMHO.

>> >--- linux-2.2.15-pre3/mm/page_alloc.c.orig	Wed Jan 19 21:32:05 2000
>> >+++ linux-2.2.15-pre3/mm/page_alloc.c	Wed Jan 19 21:42:00 2000
>> >@@ -212,7 +212,7 @@
>> > 	if (!(current->flags & PF_MEMALLOC)) {
>> > 		int freed;
>> > 
>> >-		if (nr_free_pages > freepages.min) {
>> >+		if (nr_free_pages > freepages.low) {
>> > 			if (!low_on_memory)
>> > 				goto ok_to_allocate;
>> > 			if (nr_free_pages >= freepages.high) {
>> 
>> freepages.low is unused. With this change you are using low and
>> making min unused. min seems the right name there.
>
>I consider the absense of the third boundary a HUGE bug
>in current 2.2 VM. You need three boundaries:
>
>- min:  below this boundary only ATOMIC (and GFP_HIGH?)
>        allocations can be done
>- low:  below this boundary we start swapping agressively
>- high: below this boundary we start background swapping
>        (once a second, without impact on processes), above
>        this boundary we stop swapping
>
>Current (2-border) code makes for bursty swap behaviour,
>poor handling of kernel allocations (see the complaints
>Alan got about systems which ran OOM on atomic allocations).

I missed Alan problems (probably they are hided by the l-k heavy
traffic).

high is just implemented in your way. The once a second is not enough for
RL, you need a pre-wakeup if you want to take try to pre-free memory. And
the pre-wakeup won't help in the swap case.

the need of low and min make no sense to me. You may be more happy to know
that all three freepages values in /proc are used for something but for
your machine it will make no differences. Micro tuning make no sense to
me. Only tuning that make differences in RL make sense to me.

>There is a good reason why there is a difference between
>background swapping and agressive swapping, why there is
>a separate swapout daemon, etc...

The separate swapout daemon is there only for allowing the system to free
cache and userspace memory for atomic allocations. It has nothing to do
with aggressive or not aggressive swapping.

>Performance-wise, current 2.2 went back to the stone age,
>we should do something about that.

Please run 2.2.14aa1.

>> Nobody is allowed to allocate memory without first free some
>> memory if the system is under the _min_ memory watermark.
>
>That's not the way things were meant to be. Below freepages.low
>low-priority allocations should stall, below freepages.min higher
>priority allocations should stall and only GFP_ATOMIC and
>PF_MEMALLOC allocations should be able to proceed.

The only difference between high and low prio allocation is that high prio
allocation are not triggered by an userspace action. Thus they'd better
complete trying to allocate memory even if we are low on memory. low prio
allocation are triggered by userspace so we want to fail ASAP if userspace
triggered an oom. It has nothing to do with performance and it has not to
do with the selection if somebody should stall or not.

If you are planning to change the current semantics of the GFP_* variables
please work on 2.3.x only.

Andrea

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