2015-04-03 10:04, Gonzalez Monroy, Sergio: > On 02/04/2015 14:41, Jay Rolette wrote: > > On Thu, Apr 2, 2015 at 7:55 AM, Thomas Monjalon <thomas.monjalon at > > 6wind.com> > > wrote: > > > >> 2015-04-02 19:30, jerry.lilijun at huawei.com: > >>> From: Lilijun <jerry.lilijun at huawei.com> > >>> > >>> In the function map_all_hugepages(), hugepage memory is truly allocated > >> by > >>> memset(virtaddr, 0, hugepage_sz). Then it costs about 40s to finish the > >>> dpdk memory initialization when 40000 2M hugepages are setup in host os. > >> Yes it's something we should try to reduce. > >> > > I have a patch in my tree that does the same opto, but it is commented out > > right now. In our case, 2/3's of the startup time for our entire app was > > due to that particular call - memset(virtaddr, 0, hugepage_sz). Just > > zeroing 1 byte per huge page reduces that by 30% in my tests. > > > > The only reason I have it commented out is that I didn't have time to make > > sure there weren't side-effects for DPDK or my app. For normal shared > > memory on Linux, pages are initialized to zero automatically once they are > > touched, so the memset isn't required but I wasn't sure whether that > > applied to huge pages. Also wasn't sure how hugetlbfs factored into the > > equation. > > > > Hopefully someone can chime in on that. Would love to uncomment the opto :) > > > I think the opto/patch is good ;) > > I had a look at the Linux kernel sources (mm/hugetlb.c)and at least > since 2.6.32 (minimum > Linux kernel version supported by DPDK) the kernel clears the hugepage > (clear_huge_page) > when it faults (hugetlb_no_page). > > Primary DPDK apps do clear_hugedir, clearing previously allocated > hugepages, thus triggering > hugepage faults (hugetlb_no_page) during map_all_hugepages. > > Note that even when we exit a primary DPDK app, hugepages remain > allocated, reason why > apps such as dump_cfg are able to retrieve config/memory information.
OK, thanks Sergio. So the patch should add a comment to explain page fault reason of memset and why 1 byte is enough. I think we should also consider remap_all_hugepages() function. > >> Isn't it a security hole? > >> > > Not necessarily. If the kernel pre-zeros the huge pages via CoW like normal > > pages, then definitely not. > > > > Even if the kernel doesn't pre-zero the pages, if DPDK takes care of > > properly initializing memory structures on startup as they are carved out > > of the huge pages, then it isn't a security hole. However, that approach is > > susceptible to bit rot... You can audit the code and make sure everything > > is kosher at first, but you have to worry about new code making assumptions > > about how memory is initialized. > > > >> This article speaks about "prezeroing optimizations" in Linux kernel: > >> http://landley.net/writing/memory-faq.txt > > > > I read through that when I was trying to figure out what whether huge pages > > were pre-zeroed or not. It doesn't talk about huge pages much beyond why > > they are useful for reducing TLB swaps. > > > > Jay
