Re: Overhead of highpte

Dave Hansen (haveblue@us.ibm.com)
03 Jul 2003 19:34:19 -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?

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.

-- 
Dave Hansen
haveblue@us.ibm.com

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