Re: [RFC] Patch to improve TSO limitation formula in general

Hans Petter Selesky wrote:
> On 09/05/14 23:19, Eric Joyner wrote:
> > There are some concerns if we use this with devices that ixl
> > supports:
> >
> > - The maximum fragment size is 16KB-1, which isn't a power of 2.
> >
> 
> Hi Eric,
> 
> Multiplying by powers of two are more fast, than non-powers of two.
> So
> in this case you would have to use 8KB as a maximum.
> 
Well, I'm no architecture expert, but I really doubt the CPU delay of a
non-power of 2 multiply/divide is significant related to doing smaller
TSO segments. Long ago (as in 1970s) I did work on machines where shifts
for power of 2 multiply/divide was preferable, but these days I doubt it
is going to matter??

> > - You can't get the maximum TSO size for ixl devices by multiplying
> > the
> > maximum number of fragments by the maximum size.
> > Instead the number of fragments is AFAIK unlimited, but a segment
> > can only
> > span 8 mbufs (including the [up to 3] mbufs containing the header),
> > and the
> > maximum TSO size is 256KB.
> >
> > And one question:
> >
> > - Is hdr_size_log2 supposed to be the length of the L2 header? We
> > can fit
> > 254 L2 bytes in our hardware during a TSO, so if that's the value,
> > I guess
> > that's fine, barring the it-not-being-a-power-of-2 issue.
> 
> This is the ethernet / vlan headers. It is added with the
> TCP/IP-header
> in the end.
> 
> >
> > With all that said, the 8 mbuf limit per segment issue is a TSO
> > limitation
> > that we'd like to notify the stack about, so I wonder if that could
> > be
> > incorporated along with this. Right now, our driver checks to see
> > if a
> > segment in a TSO spans more than six mbufs and then m_defrag()'s
> > the entire
> > chain if one exists; it's less than optimal but necessary to
> > prevent errors.
> 
At this time, if there is a limit of 8 TSO segments (mbufs) in a
transmit list, you will need to set:
 ifp->if_hw_tsomax = 8 * MCLBYTES - (ETHER_HDR_LEN + ETHER_VLAN_ENCAP_LEN);

- just before the call to
 ether_ifattach(ifp);

I do have an untested patch (attached in case anyone is interested) which
adds if_hw_tsomaxseg that drivers can set to their maximum number of transmit
segments (mbufs) fot TSO. This value is then used by tcp_output() to generate
appropriately sized TSO segments.
However, I'm just working on getting a way to test this patch, so I can't say
if/when it will be in head.

rick

 

> It is not impossible to move from log2 syntax to non-log2 syntax,
> hence
> the logic will be exactly the same, only that the required division
> and
> multiplication will have a bit overhead I guess.
> 
> Could you make a patch on top of my patch with the changes you think
> are
> required to fully support the ixl hardware? Or propose a new patch
> which
> also serves the MLX needs?
> 
> Thank you!
> 
> --HPS
> 
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