[dpdk-dev] Reshuffling of rte_mbuf structure.

[shesha Sreenivasamurthy (shesha)]

Ok, You are saying re-order the fields based on the configurations params. I took word "NO" in the param to eliminate. Sure, this does not require and change in the code that uses it. Will it not now boil down to same as having completely different layout definition and be more messier ?

For example: Rather than having:

#ifdef NO_TX_OFFLOAD
Struct mbuf_rte {
fieldA
field1
field2
fieldB
field4
filed5
};
#endif

#ifdef NO_MULTISEG
Struct mbuf_rte{
fieldA
field2
field1
fieldB
filed5
field4
}
#endif

We end up having

Struct mbuf_rte {
fieldA
#ifdef NO_TX_OFFLOAD
field1
field2
#endif
#ifdef NO_MULTISEG
field2
field1
#endif
fieldB
#ifdef NO_TX_OFFLOAD
field4
field5
#endif
#ifdef NO_MULTISEG
field5
field4
#endif
};



--
- Thanks
char * (*shesha) (uint64_t cache, uint8_t F00D)
{ return 0x0000C0DE; }

From: Arnon Warshavsky <arnon at qwilt.com<mailto:arnon at qwilt.com>>
Date: Monday, November 2, 2015 at 10:35 AM
To: Cisco Employee <shesha at cisco.com<mailto:shesha at cisco.com>>
Cc: Stephen Hemminger <stephen at networkplumber.org<mailto:stephen at networkplumber.org>>, "dev at dpdk.org<mailto:dev at dpdk.org>" <dev at dpdk.org<mailto:dev at dpdk.org>>
Subject: Re: [dpdk-dev] Reshuffling of rte_mbuf structure.

If NO_TX_OFFLOAD only changes the layout in terms of relative field location in cache lines, and does not eliminate the fields themselves
why should the using code be affected?

On Mon, Nov 2, 2015 at 8:30 PM, shesha Sreenivasamurthy (shesha) <shesha at cisco.com<mailto:shesha at cisco.com>> wrote:
One issue I see with optimization config options such as NO_TX_OFFLOAD, NO_MULTISEG, NO_REFCOUNT is: It is not sufficient to have those "Ifdefs" inside mbuf structure, but should be sprinkled all over the code where corresponding fields are used. This may make the code messier.

--
- Thanks
char * (*shesha) (uint64_t cache, uint8_t F00D)
{ return 0x0000C0DE; }

From: Stephen Hemminger <stephen at networkplumber.org<mailto:stephen at networkplumber.org>>
Date: Monday, November 2, 2015 at 8:24 AM
To: Arnon Warshavsky <arnon at qwilt.com<mailto:arnon at qwilt.com>>
Cc: Cisco Employee <shesha at cisco.com<mailto:shesha at cisco.com>>, "dev at dpdk.org<mailto:dev at dpdk.org>" <dev at dpdk.org<mailto:dev at dpdk.org>>
Subject: Re: [dpdk-dev] Reshuffling of rte_mbuf structure.

On Sun, 1 Nov 2015 06:45:31 +0200
Arnon Warshavsky <arnon at qwilt.com<mailto:arnon at qwilt.com>> wrote:

My 2 cents,
This was brought up in the recent user space summit, and it seems that
indeed there is no one cache lines arrangement that fits all.
OTOH multiple compile time options to suffice all flavors, would make it
unpleasant to read maintain test and debug.
(I think there was quiet a consensus in favor of reducing compile options
in general)
Currently I manage similar deviations via our own source control which I
admit to be quite a pain.
I would prefer an option of code manipulation/generation by some script
during dpdk install,
which takes the default version of rte_mbuf.h,
along with an optional user file (json,xml,elvish,whatever) defining the
structure replacements,
creating your custom version, and placing it instead of the installed copy
of rte_mbuf.h.
Maybe the only facility required from dpdk is just the ability to register
calls to such user scripts at some install stage(s), providing the mean
along with responsibility to the user.
/Arnon
On Sat, Oct 31, 2015 at 6:44 AM, shesha Sreenivasamurthy (shesha) <
shesha at cisco.com<mailto:shesha at cisco.com>> wrote:
> In Cisco, we are using DPDK for a very high speed packet processor
> application. We don't use NIC TCP offload / RSS hashing. Putting those
> fields in the first cache-line - and the obligatory mb->next datum in the
> second cache line - causes significant LSU pressure and performance
> degradation. If it does not affect other applications, I would like to
> propose reshuffling of fields so that the obligator "next" field falls in
> first cache line and RSS hashing goes to next. If this re-shuffling indeed
> hurts other applications, another idea is to make it compile time
> configurable. Please provide feedback.
>
> --
> - Thanks
> char * (*shesha) (uint64_t cache, uint8_t F00D)
> { return 0x0000C0DE; }
>

Having different layouts will be a disaster for distro's they have to choose one.
And I hate to introduce more configuration!

But we see the same issue. It would make sense if there were configuration options
for some common optimizations NO_TX_OFFLOAD, NO_MULTISEG, NO_REFCOUNT and then
the mbuf got optimized for those combinations. Seems better than config options
like LAYOUT1, LAYOUT2, ...

In this specific case, I think lots of driver could be check nb_segs == 1 and avoiding
the next field for simple packets.

Long term, I think this will be losing battle. As DPDK grows more features, the current
mbuf structure will grow there is really nothing stopping the bloat of meta data.




--

Arnon Warshavsky
Qwilt | work: +972-72-2221634 | mobile: +972-50-8583058 | arnon at qwilt.com<mailto:arnon at qwilt.com>