From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jesper Dangaard Brouer Subject: Re: order-0 vs order-N driver allocation. Was: [PATCH v10 07/12] net/mlx4_en: add page recycle to prepare rx ring for tx support Date: Mon, 8 Aug 2016 10:01:15 +0200 Message-ID: <20160808100115.143d6ed3@redhat.com> References: <1468955817-10604-1-git-send-email-bblanco@plumgrid.com> <1468955817-10604-8-git-send-email-bblanco@plumgrid.com> <1469432120.8514.5.camel@edumazet-glaptop3.roam.corp.google.com> <20160803174107.GA38399@ast-mbp.thefacebook.com> <20160804181913.26ee17b9@redhat.com> <1470381333.13693.48.camel@edumazet-glaptop3.roam.corp.google.com> <20160808021525.GA81429@ast-mbp> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Eric Dumazet , Brenden Blanco , davem@davemloft.net, netdev@vger.kernel.org, Jamal Hadi Salim , Saeed Mahameed , Martin KaFai Lau , Ari Saha , Or Gerlitz , john.fastabend@gmail.com, hannes@stressinduktion.org, Thomas Graf , Tom Herbert , Daniel Borkmann , Tariq Toukan , Mel Gorman , linux-mm , brouer@redhat.com To: Alexei Starovoitov Return-path: In-Reply-To: <20160808021525.GA81429@ast-mbp> Sender: owner-linux-mm@kvack.org List-Id: netdev.vger.kernel.org On Sun, 7 Aug 2016 19:15:27 -0700 Alexei Starovoitov wrote: > On Fri, Aug 05, 2016 at 09:15:33AM +0200, Eric Dumazet wrote: > > On Thu, 2016-08-04 at 18:19 +0200, Jesper Dangaard Brouer wrote: > > > > > I actually agree, that we should switch to order-0 allocations. > > > > > > *BUT* this will cause performance regressions on platforms with > > > expensive DMA operations (as they no longer amortize the cost of > > > mapping a larger page). > > > > > > We much prefer reliable behavior, even it it is ~1 % slower than the > > super-optimized thing that opens highways for attackers. > > +1 > It's more important to have deterministic performance at fresh boot > and after long uptime when high order-N are gone. Yes, exactly. Doing high order-N pages allocations might look good on benchmarks on a freshly booted system, but once the page allocator gets fragmented (after long uptime) then performance characteristics change. (Discussed this with Christoph Lameter during MM-summit, and he have seen issues with this kind of fragmentation in production) > > Anyway, in most cases pages are re-used, so we only call > > dma_sync_single_range_for_cpu(), and there is no way to avoid this. > > > > Using order-0 pages [1] is actually faster, since when we use high-order > > pages (multiple frames per 'page') we can not reuse the pages. > > > > [1] I had a local patch to allocate these pages using a very simple > > allocator allocating max order (order-10) pages and splitting them into > > order-0 ages, in order to lower TLB footprint. But I could not measure a > > gain doing so on x86, at least on my lab machines. > > Which driver was that? > I suspect that should indeed be the case for any driver that > uses build_skb and <256 copybreak. > > Saeed, > could you please share the performance numbers for mlx5 order-0 vs order-N ? > You mentioned that there was some performance improvement. We need to know > how much we'll lose when we turn off order-N. I'm not sure the compare will be "fair" with the mlx5 driver, because (1) the N-order page mode (MPWQE) is a hardware feature, plus (2) the order-0 page mode is done "wrongly" (by preallocating SKBs together with RX ring entries). AFAIK it is a hardware feature the MPQWE (Multi-Packet Work Queue Element) or Striding RQ, for ConnectX4-Lx. Thus, the need to support two modes in the mlx5 driver. Commit[1] 461017cb006a ("net/mlx5e: Support RX multi-packet WQE (Striding RQ)") states this gives a 10-15% performance improvement for netperf TCP stream (and ability to absorb bursty traffic). [1] https://git.kernel.org/torvalds/c/461017cb006 The MPWQE mode, uses order-5 pages. The critical question is: what happens to the performance when order-5 allocations gets slower (or impossible) due to page fragmentation? (Notice the page allocator uses a central lock for order-N pages) -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org