Re: Overhead of highpte

Martin J. Bligh (mbligh@aracnet.com)
Thu, 03 Jul 2003 19:46:24 -0700


> On Wed, 2003-07-02 at 15:53, Martin J. Bligh wrote:
>> Some people were saying they couldn't see an overhead with highpte.
>> Seems pretty obvious to me still. It should help *more* on the NUMA
>> box, as PTEs become node-local.
>>
>> The kmap_atomic is, of course, perfectly understandable. The increase
>> in the rmap functions is a bit of a mystery to me.
>>
>> M.
>>
>> Kernbench: (make -j vmlinux, maximal tasks)
>> Elapsed System User CPU
>> 2.5.73-mm3 45.38 114.91 565.81 1497.75
>> 2.5.73-mm3-highpte 46.54 130.41 566.84 1498.00
>
> OK, let's add to the mystery. Here's my run, on virtually the same
> hardware except, I don't do a bzImage. bzImage is pretty useless
> because I don't want to benchmark gzip, so I just do vmlinux. My times
> should be _faster_ than yours, right?

I do vmlinux as well.

> Elapsed: User: System: CPU:
> 2.5.73-mjb2 77.008s 937.756s 90s 1334%
> 2.5.73-mjb2-highpte 76.756s 935.464s 93.116s 1339%
>
> Yeah, system time goes up. Something funky is going on. We should have
> the same machines, except that I have twice the RAM, right? What kind
> of fs are you doing your tests on? I'm doing ramfs.

ext2.

I suspect the problem is that your gcc is such a slow piece of shit,
you're totally userspace bound. Try 2.95 (just move the /usr/bin/gcc
symlink on debian).

M.

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