Re: [Xen-devel] [PATCH net v2 1/3] xen-netback: remove pointless clause from if statement

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: netdev-owner@xxxxxxxxxxxxxxx [mailto:netdev-
> owner@xxxxxxxxxxxxxxx] On Behalf Of Sander Eikelenboom
> Sent: 28 March 2014 10:43
> To: David Laight
> Cc: Paul Durrant; netdev@xxxxxxxxxxxxxxx; Wei Liu; Ian Campbell; xen-
> devel@xxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH net v2 1/3] xen-netback: remove pointless
> clause from if statement
> 
> 
> Friday, March 28, 2014, 11:35:58 AM, you wrote:
> 
> > From: Paul Durrant
> >> > A reasonable high estimate for the number of slots required for a
> specific
> >> > message is 'frag_count + total_size/4096'.
> >> > So if that are that many slots free it is definitely ok to add the 
> >> > message.
> >> >
> >>
> >> Hmm, that may work. By total_size, I assume you mean skb->len, so that
> calculation is based on an
> >> overhead of 1 non-optimally packed slot per frag. There'd still need to be
> a +1 for the GSO 'extra'
> >> though.
> 
> > Except I meant '2 * frag_count + size/4096' :-(
> 
> > You have to assume that every fragment starts at n*4096-1 (so need
> > at least two slots). A third slot is only needed for fragments
> > longer that 1+4096+2 - but an extra one is needed for every
> > 4096 bytes after that.
> 
> He did that in his followup patch series .. that works .. for small packets
> But for larger ones it's an extremely wasteful estimate and it quickly get
> larger than the MAX_SKB_FRAGS
> we had before and even to large causing stalls. I tried doing this type of
> calculation with a CAP of
> the old  MAX_SKB_FRAGS calculation and that works.
> 

Given that works for you and caps the estimate at the old constant value I 
guess that's the modification to go for to handle this regression. I'll try to 
come up with something better for net-next.

  Paul

> However since the calculated max_needed_slots grow so fast (most of the
> time unnecessary, i put a printk in there for that and it was quite often more
> than 5 slots off),
> that is also wasteful and it uses a more complex calculation.
> 
> 
> >         David
> 
> 
> 
> 
> 
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