On Thursday, July 10, 2014 6:31:43 pm Rick Macklem wrote:
> John Baldwin wrote:
> > On Thursday, July 03, 2014 8:51:01 pm Rick Macklem wrote:
> > > Russell L. Carter wrote:
> > > >
> > > >
> > > > On 07/02/14 19:09, Rick Macklem wrote:
> > > >
> > > > > Could you please post the dmesg stuff for the network
> > > > > interface,
> > > > > so I can tell what driver is being used? I'll take a look at
> > > > > it,
> > > > > in case it needs to be changed to use m_defrag().
> > > >
> > > > em0: <Intel(R) PRO/1000 Network Connection 7.4.2> port
> > > > 0xd020-0xd03f
> > > > mem
> > > > 0xfe4a0000-0xfe4bffff,0xfe480000-0xfe49ffff irq 44 at device 0.0
> > > > on
> > > > pci2
> > > > em0: Using an MSI interrupt
> > > > em0: Ethernet address: 00:15:17:bc:29:ba
> > > > 001.000007 [2323] netmap_attach success for em0 tx
> > > > 1/1024
> > > > rx
> > > > 1/1024 queues/slots
> > > >
> > > > This is one of those dual nic cards, so there is em1 as well...
> > > >
> > > Well, I took a quick look at the driver and it does use m_defrag(),
> > > but
> > > I think that the "retry:" label it does a goto after doing so might
> > > be in
> > > the wrong place.
> > >
> > > The attached untested patch might fix this.
> > >
> > > Is it convenient to build a kernel with this patch applied and then
> > > try
> > > it with TSO enabled?
> > >
> > > rick
> > > ps: It does have the transmit segment limit set to 32. I have no
> > > idea if
> > > this is a hardware limitation.
> >
> > I think the retry is not in the wrong place, but the overhead of all
> > those
> > pullups is apparently quite severe.
> The m_defrag() call after the first failure will just barely squeeze
> the just under 64K TSO segment into 32 mbuf clusters. Then I think any
> m_pullup() done during the retry will allocate an mbuf
> (at a glance it seems to always do this when the old mbuf is a cluster)
> and prepend that to the list.
> --> Now the list is > 32 mbufs again and the bus_dmammap_load_mbuf_sg()
> will fail again on the retry, this time fatally, I think?
>
> I can't see any reason to re-do all the stuff using m_pullup() and Russell
> reported that moving the "retry:" fixed his problem, from what I understood.
Ah, I had assumed (incorrectly) that the m_pullup()s would all be nops in this
case. It seems the NIC would really like to have all those things in a single
segment, but it is not required, so I agree that your patch is fine.
> > It would be interesting to test
> > the
> > following in addition to your change to see if it improves
> > performance
> > further:
> >
> > Index: if_em.c
> > ===================================================================
> > --- if_em.c (revision 268495)
> > +++ if_em.c (working copy)
> > @@ -1959,7 +1959,9 @@ retry:
> > if (error == EFBIG && remap) {
> > struct mbuf *m;
> >
> > - m = m_defrag(*m_headp, M_NOWAIT);
> > + m = m_collapse(*m_headp, M_NOWAIT, EM_MAX_SCATTER);
> > + if (m == NULL)
> > + m = m_defrag(*m_headp, M_NOWAIT);
> Since a just under 64K TSO segment barely fits in 32 mbuf clusters,
> I'm at least 99% sure the m_collapse() will fail, but it can't hurt to
> try it. (If it supported 33 or 34, I think m_collapse() would have a
> reasonable chance of success.)
>
> Right now the NFS and krpc code creates 2 small mbufs in front of the
> read/write data clusters and I think the TCP layer adds another one.
> Even if this was modified to put it all in one cluster, I don't think
> m_collapse() would succeed, since it only copies the data up and deletes
> an mbuf from the chain if it will all fit in the preceding one. Since
> the read/write data clusters are full (except the last one), they can't
> fit in the M_TRAILINGSPACE() of the preceding one unless it is empty
> from my reading of m_collapse().
Correct, ok.
--
John Baldwin
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