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Shutemov" To: Michal Hocko Cc: Cannon Matthews , Mike Kravetz , Andrew Morton , Matthew Wilcox , David Rientjes , Greg Thelen , Salman Qazi , linux-mm@kvack.org, linux-kernel@vger.kernel.org, ak@linux.intel.com, x86@kernel.org Subject: Re: [PATCH] mm: clear 1G pages with streaming stores on x86 Message-ID: <20200309093533.qj255nrgyofmtiqz@box> References: <20200307010353.172991-1-cannonmatthews@google.com> <20200309000820.f37opzmppm67g6et@box> <20200309090630.GC8447@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200309090630.GC8447@dhcp22.suse.cz> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Mar 09, 2020 at 10:06:30AM +0100, Michal Hocko wrote: > On Mon 09-03-20 03:08:20, Kirill A. Shutemov wrote: > > On Fri, Mar 06, 2020 at 05:03:53PM -0800, Cannon Matthews wrote: > > > Reimplement clear_gigantic_page() to clear gigabytes pages using the > > > non-temporal streaming store instructions that bypass the cache > > > (movnti), since an entire 1GiB region will not fit in the cache anyway. > > > > > > Doing an mlock() on a 512GiB 1G-hugetlb region previously would take on > > > average 134 seconds, about 260ms/GiB which is quite slow. Using `movnti` > > > and optimizing the control flow over the constituent small pages, this > > > can be improved roughly by a factor of 3-4x, with the 512GiB mlock() > > > taking only 34 seconds on average, or 67ms/GiB. > > > > > > The assembly code for the __clear_page_nt routine is more or less > > > taken directly from the output of gcc with -O3 for this function with > > > some tweaks to support arbitrary sizes and moving memory barriers: > > > > > > void clear_page_nt_64i (void *page) > > > { > > > for (int i = 0; i < GiB /sizeof(long long int); ++i) > > > { > > > _mm_stream_si64 (((long long int*)page) + i, 0); > > > } > > > sfence(); > > > } > > > > > > Tested: > > > Time to `mlock()` a 512GiB region on broadwell CPU > > > AVG time (s) % imp. ms/page > > > clear_page_erms 133.584 - 261 > > > clear_page_nt 34.154 74.43% 67 > > > > Some macrobenchmark would be great too. > > > > > An earlier version of this code was sent as an RFC patch ~July 2018 > > > https://patchwork.kernel.org/patch/10543193/ but never merged. > > > > Andi and I tried to use MOVNTI for large/gigantic page clearing back in > > 2012[1]. Maybe it can be useful. > > > > That patchset is somewhat more complex trying to keep the memory around > > the fault address hot in cache. In theory it should help to reduce latency > > on the first access to the memory. > > > > I was not able to get convincing numbers back then for the hardware of the > > time. Maybe it's better now. > > > > https://lore.kernel.org/r/1345470757-12005-1-git-send-email-kirill.shutemov@linux.intel.com > > Thanks for the reminder. I've had only a very vague recollection. Your > series had a much wider scope indeed. Since then we have gained > process_huge_page which tries to optimize normal huge pages. > > Gigantic huge pages are a bit different. They are much less dynamic from > the usage POV in my experience. Micro-optimizations for the first access > tends to not matter at all as it is usually pre-allocation scenario. On > the other hand, speeding up the initialization sounds like a good thing > in general. It will be a single time benefit but if the additional code > is not hard to maintain then I would be inclined to take it even with > "artificial" numbers state above. There really shouldn't be other downsides > except for the code maintenance, right? I cannot think of any, no. -- Kirill A. Shutemov