Re: [RFC 0/8] mbuf: structure reorganization

On Tue, Feb 21, 2017 at 03:20:23PM +0100, Morten Brørup wrote:

Comments at the end.

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-----Original Message-----
From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
Sent: Thursday, February 16, 2017 5:14 PM
To: Bruce Richardson
Cc: Ananyev, Konstantin; dev@dpdk.org
Subject: Re: [dpdk-dev] [RFC 0/8] mbuf: structure reorganization

On Thu, 16 Feb 2017 15:46:19 +0000, Bruce Richardson
[off-list ref] wrote:

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On Thu, Feb 16, 2017 at 02:48:07PM +0100, Olivier Matz wrote:

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Hi Konstantin,

Thanks for the feedback.
Comments inline.


On Mon, 6 Feb 2017 18:41:27 +0000, "Ananyev, Konstantin"
[off-list ref] wrote:

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Hi Olivier,
Looks good in general, some comments from me below.
Thanks
Konstantin

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The main changes are:
- reorder structure to increase vector performance on some
non-ia platforms.
- add a 64bits timestamp field in the 1st cache line

Wonder why it deserves to be in first cache line?
How it differs from seqn below (pure SW stuff right now).

In case the timestamp is set from a NIC value, it is set in the Rx
path. So that's why I think it deserve to be located in the 1st
cache line.

As you said, the seqn is a pure sw stuff right: it is set in a lib,
not in a PMD rx path.

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- m->next, m->nb_segs, and m->refcnt are always initialized for
mbufs in the pool, avoiding the need of setting m->next

(located

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in the 2nd cache line) in the Rx path for mono-segment packets.
- change port and nb_segs to 16 bits

Not that I am completely against it, but changing nb_segs to 16
bits seems like an overkill to me.
I think we can keep and extra 8bits for something more useful in
future.

In my case, I use the m->next field to chain more than 256 segments
for L4 socket buffers. It also updates nb_seg that can overflow.
It's not a big issue since at the end, nb_seg is decremented for
each segment. On the other hand, if I enable some sanity checks on
mbufs, it complains because the number of segments is not equal to
nb_seg.

There is also another use case with fragmentation as discussed
recently: http://dpdk.org/dev/patchwork/patch/19819/

Of course, dealing with a long mbuf list is not that efficient, but
the application can maintain another structure to accelerate the
access to the middle/end of the list.

Finally, we have other ideas to get additional 8 bits if required

in

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the future, so I don't think it's really a problem.

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- move seqn in the 2nd cache line

Things discussed but not done in the patchset:
- move refcnt and nb_segs to the 2nd cache line: many drivers
sets them in the Rx path, so it could introduce a performance
regression, or

I wonder can refcnt only be moved into the 2-nd cacheline?
As I understand thanks to other change (from above) m->refcnt

will

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already be initialized, so RX code don't need to touch it.
Though yes, it still would require changes in all PMDs.

Yes, I agree, some fields could be moved in the 2nd cache line once
all PMDs stop to write them in RX path. I propose to issue some
guidelines to PMD maintainers at the same time the patchset is
pushed. Then we can consider changing it in a future version, in
case we need more room in the 1st mbuf cache line.

If we are changing things, we should really do all that now, rather
than storing up future breaks to mbuf. Worst case, we should plan for
it immediately after the release where we make these changes. Have

two

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releases that break mbuf immediately after each other - and flagged

as

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such, but keep it stable thereafter. I don't like having technical
debt on mbuf just after we supposedly "fix" it.

I think there is no need to do this change now. And I don't feel good
with the idea of having a patchset that updates all the PMDs to remove
the access to a field because it moved to the 2nd cache line
(especially thinking about vector PMDs).

That's why I think the plan could be:
- push an updated version of this patchset quickly
- advertise to PMD maintainers "you don't need to set the m->next,
  m->refcnt, and m->nb_segs in the RX path, please update your drivers"
- later, if we need more room in the 1st cache line of the mbuf, we
  can move refcnt and nb_seg, probably without impacting the
  performance.


Olivier

I suppose you mean that PMDs don't need to /initialize/ m->next, m->refcnt and m->nb_segs.

Forgive my ignorance, and this is wild speculation, but: Would a PMD not need to set m->next and m->nb_segs if it receives a jumbogram larger than an mbuf packet buffer? And if this is a realistic use case, these fields actually do belong in the 1st cache line. PMD developers please chime in.

Yes, it would. However, this is not really fast-path processing. If we
assume a 2GHz CPU, for 64-byte packets, a core has 34 cycles to process
each packet to achieve 40G line rate. For a packet of size 2k - the
normal size it would need to hit to overflow a buffer, unless you are
using small buffers - the core has 827 cycles per packet. Therefore, in
the latter case, with big packets, the core can afford the hit of
accessing the second cacheline.

/Bruce