On Sun, Nov 3, 2013 at 11:36 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote: > So I think it all really depends on the hit/miss cost difference. It makes > little sense to add a more complex scheme if it washes out most of the > benefits! > > Also note the historic context: the _original_ mmap_cache, that I > implemented 16 years ago, was a front-line cache to a linear list walk > over all vmas (!). > > Today we have the vma rbtree, which is self-balancing and a lot faster > than your typical linear list walk search ;-) > > So I'd _really_ suggest to first examine the assumptions behind the cache, > it being named 'cache' and it having a hit rate does in itself not > guarantee that it gives us any worthwile cost savings when put in front of > an rbtree ... Agree. We have made the general case a lot faster, and caches in front of it may not pull their weight anymore - the fact that we are wondering how to even measure that, to me, means that we probably shouldn't even bother. That's what I did when I implemented the augmented rbtree to search for allocatable spaces between vmas: I removed the cache for the last used gap, and nobody has complained about it since. Absent some contrary data, I would actually prefer we remove the mmap_cache as well. And if a multiple-entry cache is necessary, I would also prefer it to be LRU type rather than something ad-hoc (if there is a benefit to caching the largest VMA, then LRU would capture that as well...) -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>