On Monday 18 March 2002 10:21, Nick Craig-Wood wrote:
> There has been some discussion on the linux kernel mailing list about
> providing 2 MB pages (instead of 4kB ones) to user space for the use
> of database or scientific calculations.
> It seems to me that prime95/mprime would benefit from this enormously
> - it should reduce the TLB thrashing to practically zero and hence
> speed up mprime by some unknown amount.
> Is this true?  Should we put in our plea to the developers?

Other people may and probably will disagree, but I think it will make very 
little if any difference for most applications.

The point is that mprime should normally be running on a system in a way 
which means that all its active data pages are in memory. Having active data 
paged out will cause a hideous performance hit.

If the active data is already memory resident, TLB thrashing is not going to 
be an issue.

Applications written in such a way that rarely-accessed data is stored in 
virtual memory with the intention that the OS allows it to be paged out are a 
different matter - larger page sizes would undoubtedly help those, at least 
to some extent.

If the page size is going to be changed at all, there is a lot to be said for 
using the same size pages as AGP hardware - 4MB I think - there have already 
been some issues on some Athlon (K7) architecture linux systems caused by 
incorrect mapping between linux virtual pages and AGP address space; 
obviously using the same page size removes this source of confusion.

One factor with shifting to a much larger page size is a corresponding 
decrease in the number of pages available to the system - a 32 MByte system 
will have only 8 4MB pages resident in real memory at any one time. Since 
page access rules are often used to protect data from accidental modification 
by rogue pointers etc., a big reduction in system physical page count is a 
distinctly mixed blessing.

As a project I don't think we need to make reccomendations one way or the 
other. As an individual I would say either go with AGP or stick with the 
status quo; and I think the status quo is better suited to systems with small 
to moderate amounts of physical memory (certainly those with less than 256 

Brian Beesley

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