Eric Dumazet
2013-Sep-04 21:00 UTC
Re: [PATCH] net/core: Order-3 frag allocator causes SWIOTLB bouncing under Xen
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 <zoltan.kiss@citrix.com> > --- > 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.