linux-kernel - Re: [patch 0/7] [RFC] SLUB: Improve allocpercpu to reduce per cpu access overhead

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Date:	Fri, 02 Nov 2007 00:00:10 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Christoph Lameter <clameter@....com>
CC:	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...ymtl.ca, penberg@...helsinki.fi
Subject: Re: [patch 0/7] [RFC] SLUB: Improve allocpercpu to reduce per cpu
 access overhead

Christoph Lameter a écrit :
> On Thu, 1 Nov 2007, David Miller wrote:
> 
>> From: Christoph Lameter <clameter@....com>
>> Date: Thu, 1 Nov 2007 15:15:39 -0700 (PDT)
>>
>>> After boot is complete we allow the reduction of the size of the per cpu 
>>> areas . Lets say we only need 128k per cpu. Then the remaining pages will
>>> be returned to the page allocator.
>> You don't know how much you will need.  I exhausted the limit on
>> sparc64 very late in the boot process when the last few userland
>> services were starting up.
> 
> Well you would be able to specify how much will remain. If not it will 
> just keep the 2M reserve around.
> 
>> And if I subsequently bring up 100,000 IP tunnels, it will exhaust the
>> per-cpu allocation area.
> 
> Each tunnel needs 4 bytes per cpu?

well, if we move last_rx to a percpu var, we need  8 bytes of percpu space per 
net_device :)

> 
>> You have to make it fully dynamic, there is no way around it.
> 
> Na. Some reasonable upper limit needs to be set. If we set that to say 
> 32Megabytes and do the virtual mapping then we can just populate the first 
> 2M and only allocate the remainder if we need it. Then we need to rely on 
> Mel's defrag stuff though defrag memory if we need it.

If a 2MB page is not available, could we revert using 4KB pages ? (like 
vmalloc stuff), paying an extra runtime overhead of course.


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