Re: Memory performance / Cache problem

[epsi@gmx.de]

> On Wednesday 14 October 2009 17:48:39 ext epsi@gmx.de wrote:
> > Mem clock is both times 166MHz. I don't know whether are differences in
> > cycle access and timing, but memclock is fine.
> >
> > Following Siarhei hints of initialize the buffers (around 1.2 MByte
> each)
> > I get different results in 22kernel for use of
> > malloc alone
> > memcpy =   473.764, loop4 =   448.430, loop1 =   102.770, rand =   
> 29.641
> > calloc alone
> > memcpy =   405.947, loop4 =   361.550, loop1 =    95.441, rand =   
> 21.853
> > malloc+memset:
> > memcpy =   239.294, loop4 =   188.617, loop1 =    80.871, rand =    
> 4.726
> >
> > In 31kernel all 3 measures are about the same (unfortunatly low) level
> of
> > malloc+memset in 22.
> >
> > First of all: What performance can be expected?
> > Does 22 make failures if it is so much faster?
> > Can the later kernels get a boost in memory handling?
> 
> What you see is just a (fake) performance boost because you have a single
> physical page shared between all the virtual pages in the source buffer.
> So
> you get no cache misses on read operations and everything seems fast.
> 
> This is unlikely to happen on real use, and it does not reflect real
> memory
> performance. So the benchmark is inadequate.

> 
> You can get some basic information here:
> http://en.wikipedia.org/wiki/Copy-on-write
> 
> Regarding the difference in behavior between .22 and recent kernels. It
> may be
> some regression in copy-on-write implementation, or just some change done
> on
> purpose. That is assuming that the userspace stuff was identical in both
> tests.
> 

Ok, understand the difference if the memory is uninitialised.
But why there is the difference in "malloc + memset" and "calloc"?
In both cases the memory will be cleared.


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