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 MBytes). Regards Brian Beesley _________________________________________________________________________ Unsubscribe & list info -- http://www.ndatech.com/mersenne/signup.htm Mersenne Prime FAQ -- http://www.tasam.com/~lrwiman/FAQ-mers
