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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: David Laight [mailto:David.Laight@xxxxxxxxxx]
> Sent: 28 March 2014 10:36
> To: Paul Durrant; Sander Eikelenboom
> Cc: 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
> 
> 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' :-(
> 

And that's the pessimal estimation that's failing for Sander :-(

  Paul

> 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.
> 
>       David
> 
> 


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