From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [PATCH] net/core: Order-3 frag allocator causes SWIOTLB bouncing under Xen Date: Wed, 4 Sep 2013 17:11:10 -0400 Message-ID: <20130904211110.GA17758@phenom.dumpdata.com> References: <1378327638-23956-1-git-send-email-zoltan.kiss@citrix.com> <1378328440.7360.124.camel@edumazet-glaptop> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Zoltan Kiss , "David S. Miller" , Eric Dumazet , Neil Horman , Li Zefan , Eliezer Tamir , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, malcolm.crossley@citrix.com, david.vrabel@citrix.com, xen-devel@lists.xen.org To: Eric Dumazet Return-path: Content-Disposition: inline In-Reply-To: <1378328440.7360.124.camel@edumazet-glaptop> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Sep 04, 2013 at 02:00:40PM -0700, Eric Dumazet wrote: > On Wed, 2013-09-04 at 21:47 +0100, Zoltan Kiss wrote: > > THIS PATCH IS NOT INTENDED TO BE UPSTREAMED, IT HAS ONLY INFORMING PURPOSES! > > > > I've noticed a performance regression with upstream kernels when used as Dom0 > > under Xen. The classic kernel can utilize the whole bandwidth of a 10G NIC > > (ca. 9.3 Gbps), but upstream can reach only ca. 7 Gbps. I found that it > > happens because SWIOTLB has to do double buffering. The per task frag > > allocator introduced in 5640f7 creates 32 kb frags, which are not contiguous > > in mfn space. > > This patch provides a workaround by going back to the old way. The possible > > ideas came up to solve this: > > > > * make sure Dom0 memory is contiguous: it sounds trivial, but doesn't work with > > driver domains, and there are lots of situations where this is not possible. > > * use PVH Dom0: so we will have IOMMU. In the future sometime. > > * use IOMMU with PV Dom0: this seems to happen earlier. > > > > Signed-off-by: Zoltan Kiss > > --- > > net/core/sock.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/net/core/sock.c b/net/core/sock.c > > index 2c097c5..854a0ea 100644 > > --- a/net/core/sock.c > > +++ b/net/core/sock.c > > @@ -1812,7 +1812,7 @@ struct sk_buff *sock_alloc_send_skb(struct sock *sk, unsigned long size, > > EXPORT_SYMBOL(sock_alloc_send_skb); > > > > /* On 32bit arches, an skb frag is limited to 2^15 */ > > -#define SKB_FRAG_PAGE_ORDER get_order(32768) > > +#define SKB_FRAG_PAGE_ORDER get_order(4096) > > > > Well, this hack is not new... > > We have dev->gso_max_size and dev->gso_max_segs > > We also have in net-next sk_pacing_rate and dynamic TSO sizing. > > Maybe you could add proper infrastructure to deal with Xen limitations. I think Ian posted at some point an sysctl patch for that (more for debugging that anything else). And it kind of stalled: http://lists.xen.org/archives/html/xen-devel/2012-10/msg01832.html Is that what you mean by proper infrastructure ? Oh wait, did you mean via dev and not the whole system wide sysctl? > > >