On Thu, 2012-10-25 at 14:16 -0400, Chris Metcalf wrote: > On 10/25/2012 1:51 PM, Ben Hutchings wrote: > > On Thu, 2012-10-25 at 13:25 -0400, Chris Metcalf wrote: > >> This change correctly computes the header length and data length in > >> the fragments to avoid a bug where we would end up with extremely > >> slow performance. Also adopt use of skb_frag_size() accessor. > > [...] > > > > By the way, since you're doing soft-TSO you should probably set > > net_device::gso_max_segs, as explained in: > > > > commit 30b678d844af3305cda5953467005cebb5d7b687 > > Author: Ben Hutchings <bhutchi...@solarflare.com> > > Date: Mon Jul 30 15:57:00 2012 +0000 > > > > net: Allow driver to limit number of GSO segments per skb > > We currently have a hard limit of 2048 equeue entries (effectively, > segments) per interface. The commit message suggests 861 is the largest > number we're likely to see, so I think we're OK from a correctness point of > view. > > But, perhaps, we could end up with multiple cores trying to push > separate flows each with this tiny MSS issue, and they would then be > contending for the 2048 equeue entries, and performance might suffer. I > don't have a good instinct on what value we should choose to set here; I > see that sfc uses 100. > > So, we could do nothing since it seems we're technically safe; we could say > 2048 to be explicit; we could pick a random fraction of the full size to > help avoid contention effects, like 1024 or 512; or we could mimic sfc and > just say 100. What do you think?
You need at least 2 descriptors per segment, plus 1 for each fragment boundary, so the maximum would be something like (2048 - MAX_SKB_FRAGS) / 2 ~= 1015. So the worst-case skb can fit in the queue, but it requires so much of the queue space that the queue might not become sufficiently empty before the watchdog timer fires. There's a simple way to test this - configure the 'attacker' host and interface like this: # ip link set $IFACE mtu 128 # sysctl net.ipv4.route.min_adv_mss=88 and then use something like netperf to generate a high bandwidth stream of TCP traffic toward it from the tilegx interface. Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
