[ofa-general] Re: Can Mellanox cards handle scatter gather I/O?

[Michael S. Tsirkin]

> Quoting Kent Baxley <kbaxley at redhat.com>:
> Subject: Can Mellanox cards handle scatter gather I/O?
> 
> 
> Evidently, we're seeing performance problems when a card on one side
> is capable of scatter gather IO but the mellanox driver is not able to
> handle it. Then a whole lot of time is spent gathering the data into
> one skbuff.

I guess you are speaking about a router configuration where
you stick 2 network cards in the same machine - right?

> This is currently being observed on RHEL4.5 with OFED 1.2.
> 
> Is the Mellanox card capable of doing scatter gather IO and this just
> a driver limitation or is the mellanox card just not able to do scatter
> gather IO?

AFAIK all Mellanox cards are capable of s/g IO in hardware.

However, speaking of IPoIB, the code in
net/core/dev.c below would just remove NETIF_F_SG immediately:

        if ((dev->features & NETIF_F_SG) &&
            !(dev->features & NETIF_F_ALL_CSUM)) {
                printk(KERN_NOTICE "%s: Dropping NETIF_F_SG since no checksum feature.\n",
                       dev->name);
                dev->features &= ~NETIF_F_SG;
        }

we can work around this code in OFED, but I think it would
make sense to first have a discussion on netdev and lkml list about why is
this code there in the first place.

-- 
MST