Re: x86: 4kstacks default

[Andi Kleen]

Daniel Hazelton wrote:

> Andi, you're the only one I've seen seriously pounding the "50k threads" 
> thing. I don't think anyone is really fooled by the straw-man, so I'd
> suggest you drop it.

Ok, perhaps we can settle this properly. Like historicans. We study the
original sources.

The primary resource is the original commit adding the 4k stack code.
You cannot find this in latest git because it predates 2.6.12, but it is
available in one of the historic trees imported from BitKeeper like
git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

Here's the log:
>>
commit 95f238eac82907c4ccbc301cd5788e67db0715ce
Author: Andrew Morton <akpm@xxxxxxxx>
Date:   Sun Apr 11 23:18:43 2004 -0700

    [PATCH] ia32: 4Kb stacks (and irqstacks) patch

    From: Arjan van de Ven <arjanv@xxxxxxxxxx>

    Below is a patch to enable 4Kb stacks for x86. The goal of this is to

    1) Reduce footprint per thread so that systems can run many more threads
       (for the java people)

    2) Reduce the pressure on the VM for order > 0 allocations. We see
real life
       workloads (granted with 2.4 but the fundamental fragmentation
issue isn't
       solved in 2.6 and isn't solvable in theory) where this can be a
problem.
       In addition order > 0 allocations can make the VM "stutter" and
give more
       latency due to having to do much much more work trying to defragment

...
<<

This gives us two reasons as you can see, one of them many threads
and another mostly only relevant to 2.4

Now I was also assuming that nobody took (1) really serious and
attacked (2) in earlier thread; in particular in

http://article.gmane.org/gmane.linux.kernel/665584

>>
Actually the real reason the 4K stacks were introduced IIRC was that
the VM is not very good at allocation of order > 0 pages and that only
using order 0 and not order 1 in normal operation prevented some stalls.

This rationale also goes back to 2.4 (especially some of the early 2.4
VMs were not very good) and the 2.6 VM is generally better and on
x86-64 I don't see much evidence that these stalls are a big problem
(but then x86-64 also has more lowmem).
<<

This was corrected by Ingo who was one of the primary authors of the patch:

http://thread.gmane.org/gmane.linux.kernel/665420:

>>
no, the primary motivation Arjan and me started working on 4K stacks and
implemented it was what Denys mentioned: i had a testcase that ran
50,000 threads before it ran out of memory - i wanted it to run 100,000
threads. The improved order-0 behavior was just icing on the cake.

	Ingo
<<

and then from Arjan:

http://thread.gmane.org/gmane.linux.kernel/665420

>>
> no, the primary motivation Arjan and me started working on 4K stacks
> and implemented it was what Denys mentioned: i had a testcase that

well that and the fact that RH had customers who had major issues at
fewer threads
with 8Kb versus fragmentation.
<<

So both the primary authors of the patch state that 50k threads
was the main reason. I didn't believe it at first either, but after
these forceful corrections I do now.

You're totally wrong when you call it a straw man.

-Andi

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