On 12 Aug 2014, at 19:09, John Baldwin <j...@freebsd.org> wrote: > OTOH, I have actually seen junk profiling _improve_ performance in certain > cases as it forces promotion of allocated pages to superpages since all pages > are dirtied. (I have a local hack that adds a new malloc option to > explicitly > memset() new pages allocated via mmap() that gives the same benefit without > the junking overheadon each malloc() / free(), but it does increase physical > RAM usage.)
Do you get the same effect by adding MAP_ALIGNED_SUPER | MAP_PREFAULT_READ to the mmap() call in jemalloc? I've been meaning to try the latter on BERI, as we spend a lot of time bouncing back and forth between user code and the TLB miss handlers. Given that jemalloc asks for memory in 8MB chunks (I think via a single mmap call, although I'm not 100% certain), MAP_ALIGNED_SUPER should have little impact on any platform. MAP_PREFAULT_READ may cause problems on machines with limited RAM and no swap (I don't know if the VM subsystem knows that it can safely discard a zero'd page that has been read but not written - I'd hope so, but it's been a while since I read that code). It might be that we can make jemalloc autotune whether to use MAP_PREFAULT_READ depending on some heuristic. I wonder if something as simple as 'turn it on after the first mmap call' would be enough: programs that don't use more than 8MB of RAM won't prefault, but after that the wasted physical memory becomes an increasingly small percentage. David _______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
