On Wed, Nov 4, 2015 at 12:00 PM, Shaohua Li <shli@xxxxxxxxxx> wrote: > Compared to MADV_DONTNEED, MADV_FREE's lazy memory free is a huge win to reduce > page fault. But there is one issue remaining, the TLB flush. Both MADV_DONTNEED > and MADV_FREE do TLB flush. TLB flush overhead is quite big in contemporary > multi-thread applications. In our production workload, we observed 80% CPU > spending on TLB flush triggered by jemalloc madvise(MADV_DONTNEED) sometimes. > We haven't tested MADV_FREE yet, but the result should be similar. It's hard to > avoid the TLB flush issue with MADV_FREE, because it helps avoid data > corruption. > > The new proposal tries to fix the TLB issue. We introduce two madvise verbs: > > MARK_FREE. Userspace notifies kernel the memory range can be discarded. Kernel > just records the range in current stage. Should memory pressure happen, page > reclaim can free the memory directly regardless the pte state. > > MARK_NOFREE. Userspace notifies kernel the memory range will be reused soon. > Kernel deletes the record and prevents page reclaim discards the memory. If the > memory isn't reclaimed, userspace will access the old memory, otherwise do > normal page fault handling. > > The point is to let userspace notify kernel if memory can be discarded, instead > of depending on pte dirty bit used by MADV_FREE. With these, no TLB flush is > required till page reclaim actually frees the memory (page reclaim need do the > TLB flush for MADV_FREE too). It still preserves the lazy memory free merit of > MADV_FREE. > > Compared to MADV_FREE, reusing memory with the new proposal isn't transparent, > eg must call MARK_NOFREE. But it's easy to utilize the new API in jemalloc. > > We don't have code to backup this yet, sorry. We'd like to discuss it if it > makes sense. Sorry to be so slow to reply here! As Minchan mentioned, this is very similar in concept to the volatile ranges work Minchan and I tried to push for a few years. Here's some of the coverage (in reverse chronological order) https://lwn.net/Articles/602650/ https://lwn.net/Articles/592042/ https://lwn.net/Articles/590991/ http://permalink.gmane.org/gmane.linux.kernel.mm/98848 http://permalink.gmane.org/gmane.linux.kernel.mm/98676 https://lwn.net/Articles/522135/ https://lwn.net/Kernel/Index/#Volatile_ranges If you are interested in reviving the patch set, I'd love to hear about it. I think its a really compelling feature for kernel right-sizing of userspace caches. thanks -john -- 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>