Re: [PATCH] Re: 2G memory split

["Jeff V. Merkey" <jmerkey@wolfmountaingroup.com>]

Lennart Sorensen wrote:

> On Tue, Jan 10, 2006 at 09:30:58AM -0700, Jeff V. Merkey wrote:
>  
>
> > Bernd Eckenfels wrote:
> >
> > Here are the patches I use for the splitting.  They work well.    The 
> > methods employed in Red Hat ES are far better and I am surprised
> > no one has simply integrated those patches into the kernel which are 4GB 
> > / 4GB  kernel/user. 
> >    
>
> I was under the impression the 4G/4G split had some non negligable
> performance penalties compared to the other options.
>  

It does, but from my testing, I/O performance and app performance seems 
negligible. I run the highest performing app/driver
on Linux for disk and network I/O loading and ES3 and ES4 are just as 
performant with 4:4 as FC2, FC3, and FC4 with
3:1.

I am now able to capture 4 x gigabit segments with 3:1 at sustained 
stream to disk rates of 497 MB/S
and 1 x 10Gbe at 517 MB/S stream to disk. I see no appreciable 
performance differences 3:1 vs. 4:4. Modern Xeon
processors have gotten a lot better dealing with TLB invalidation. I 
suppose applications that remap
memory all over the place or that do tons of swapping would see some 
penalty, and I do see some
performance degredation when user space apps start swapping, but it's 
difficult to quantify how much is related
to disk I/O latency vs. TLB overhear. Most TLB flushes will cost you 150 
clocks over time as the TLB reloads
itself.

Jeff

> Len Sorensen
>
>  

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