linux-kernel - Re: #tj-percpu has been rebased

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Date:	Sat, 14 Feb 2009 11:10:26 +0900
From:	Tejun Heo <tj@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>, cpw@....com
Subject: Re: #tj-percpu has been rebased

Hello,

H. Peter Anvin wrote:
> Okay, let's think about this a bit.
> 
> At least for x86, there are two cases:
> 
> - 32 bits.  The vmalloc area is *extremely* constrained, and has the
> same class of fragmentation issues as main memory.  In fact, it might
> have *more* just by virtue of being larger.

We can go for smaller chunks but I don't really see any perfect
solution here.  If a machine is doing 16 way SMP on 32bit, it's not
gonna scale very well anyway.

> - 64 bits.  At this point, we have with current memory sizes(*) an
> astronomically large virtual space.  Here we have no real problem
> allocating linearly in virtual space, either by giving each CPU some
> very large hunk of virtual address space (which means each percpu area
> is contiguous in virtual space) or by doing large contiguous allocations
> out of another range.
> 
> It doesn't seem to make sense to me at first glance to be any advantage
> to interlacing the CPUs.  Quite on the contrary, it seems to utterly
> preclude ever doing PMDs with a win, since (a) you'd be allocating real
> memory for CPUs which aren't actually there and (b) you'd have the wrong
> NUMA associativity.

For (a), we can do hotplug online/offline thing for dynamic areas if
necessary.  (b) why would it have the wrong NUMA associativity?

Thanks.

-- 
tejun
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