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

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Friday, March 28, 2014, 11:47:04 AM, you wrote:

>> -----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.

OK send the patch and i will retest immediately just to make sure.

>   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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