[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] net/core: Order-3 frag allocator causes SWIOTLB bouncing under Xen
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 <zoltan.kiss@xxxxxxxxxx> > > --- > > 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? > > > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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