Re: [dpdk-dev] [PATCH] eal: decrease the memory init time with many hugepages setup

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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@6wind.com>
> > wrote:
> >
> >> 2015-04-02 19:30, jerry.lilijun@huawei.com:
> >>> From: Lilijun <jerry.lilijun@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