[dpdk-dev] mbuf changes

[Adrien Mazarguil]

On Fri, Oct 28, 2016 at 04:11:45PM +0200, Morten Brørup wrote:
> Comments at the end.
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Pattan, Reshma
> > Sent: Friday, October 28, 2016 3:35 PM
> > To: Olivier Matz
> > Cc: dev at dpdk.org; Morten Brørup
> > Subject: Re: [dpdk-dev] mbuf changes
> > 
> > Hi Olivier,
> > 
> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Olivier Matz
> > > Sent: Tuesday, October 25, 2016 1:49 PM
> > > To: Richardson, Bruce <bruce.richardson at intel.com>; Morten Brørup
> > > <mb at smartsharesystems.com>
> > > Cc: Adrien Mazarguil <adrien.mazarguil at 6wind.com>; Wiles, Keith
> > > <keith.wiles at intel.com>; dev at dpdk.org; Oleg Kuporosov
> > > <olegk at mellanox.com>
> > > Subject: Re: [dpdk-dev] mbuf changes
> > >
> > >
> > >
> > > On 10/25/2016 02:45 PM, Bruce Richardson wrote:
> > > > On Tue, Oct 25, 2016 at 02:33:55PM +0200, Morten Brørup wrote:
> > > >> Comments at the end.
> > > >>
> > > >> Med venlig hilsen / kind regards
> > > >> - Morten Brørup
> > > >>
> > > >>> -----Original Message-----
> > > >>> From: Bruce Richardson [mailto:bruce.richardson at intel.com]
> > > >>> Sent: Tuesday, October 25, 2016 2:20 PM
> > > >>> To: Morten Brørup
> > > >>> Cc: Adrien Mazarguil; Wiles, Keith; dev at dpdk.org; Olivier Matz;
> > > >>> Oleg Kuporosov
> > > >>> Subject: Re: [dpdk-dev] mbuf changes
> > > >>>
> > > >>> On Tue, Oct 25, 2016 at 02:16:29PM +0200, Morten Brørup wrote:
> > > >>>> Comments inline.
> > > >>>>
> > > >>>>> -----Original Message-----
> > > >>>>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Bruce
> > > >>>>> Richardson
> > > >>>>> Sent: Tuesday, October 25, 2016 1:14 PM
> > > >>>>> To: Adrien Mazarguil
> > > >>>>> Cc: Morten Brørup; Wiles, Keith; dev at dpdk.org; Olivier Matz;
> > > >>>>> Oleg Kuporosov
> > > >>>>> Subject: Re: [dpdk-dev] mbuf changes
> > > >>>>>
> > > >>>>> On Tue, Oct 25, 2016 at 01:04:44PM +0200, Adrien Mazarguil
> > wrote:
> > > >>>>>> On Tue, Oct 25, 2016 at 12:11:04PM +0200, Morten Brørup wrote:
> > > >>>>>>> Comments inline.
> > > >>>>>>>
> > > >>>>>>> Med venlig hilsen / kind regards
> > > >>>>>>> - Morten Brørup
> > > >>>>>>>
> > > >>>>>>>
> > > >>>>>>>> -----Original Message-----
> > > >>>>>>>> From: Adrien Mazarguil [mailto:adrien.mazarguil at 6wind.com]
> > > >>>>>>>> Sent: Tuesday, October 25, 2016 11:39 AM
> > > >>>>>>>> To: Bruce Richardson
> > > >>>>>>>> Cc: Wiles, Keith; Morten Brørup; dev at dpdk.org; Olivier Matz;
> > > >>>>>>>> Oleg Kuporosov
> > > >>>>>>>> Subject: Re: [dpdk-dev] mbuf changes
> > > >>>>>>>>
> > > >>>>>>>> On Mon, Oct 24, 2016 at 05:25:38PM +0100, Bruce Richardson
> > > >>> wrote:
> > > >>>>>>>>> On Mon, Oct 24, 2016 at 04:11:33PM +0000, Wiles, Keith
> > > >>> wrote:
> > > >>>>>>>> [...]
> > > >>>>>>>>>>> On Oct 24, 2016, at 10:49 AM, Morten Brørup
> > > >>>>>>>> <mb at smartsharesystems.com> wrote:
> > > >>>>>>>> [...]
> > > >>>>
> > > >>>>>>>>> One other point I'll mention is that we need to have a
> > > >>>>>>>>> discussion on how/where to add in a timestamp value into
> > > >>> the
> > > >>>>>>>>> mbuf. Personally, I think it can be in a union with the
> > > >>>>> sequence
> > > >>>>>>>>> number value, but I also suspect that 32-bits of a
> > > >>> timestamp
> > > >>>>>>>>> is not going to be enough for
> > > >>>>>>>> many.
> > > >>>>>>>>>
> > > >>>>>>>>> Thoughts?
> > > >>>>>>>>
> > > >>>>>>>> If we consider that timestamp representation should use
> > > >>>>> nanosecond
> > > >>>>>>>> granularity, a 32-bit value may likely wrap around too
> > > >>> quickly
> > > >>>>>>>> to be useful. We can also assume that applications
> > requesting
> > > >>>>>>>> timestamps may care more about latency than throughput, Oleg
> > > >>>>> found
> > > >>>>>>>> that using the second cache line for this purpose had a
> > > >>>>> noticeable impact [1].
> > > >>>>>>>>
> > > >>>>>>>>  [1] http://dpdk.org/ml/archives/dev/2016-
> > October/049237.html
> > > >>>>>>>
> > > >>>>>>> I agree with Oleg about the latency vs. throughput importance
> > > >>>>>>> for
> > > >>>>> such applications.
> > > >>>>>>>
> > > >>>>>>> If you need high resolution timestamps, consider them to be
> > > >>>>> generated by the NIC RX driver, possibly by the hardware itself
> > > >>>>> (http://w3new.napatech.com/features/time-precision/hardware-
> > time
> > > >>>>> - stamp), so the timestamp belongs in the first cache line. And
> > > >>>>> I am proposing that it should have the highest possible
> > > >>>>> accuracy, which makes the value hardware dependent.
> > > >>>>>>>
> > > >>>>>>> Furthermore, I am arguing that we leave it up to the
> > > >>> application
> > > >>>>>>> to
> > > >>>>> keep track of the slowly moving bits (i.e. counting whole
> > > >>>>> seconds, hours and calendar date) out of band, so we don't use
> > > >>>>> precious
> > > >>> space
> > > >>>>> in the mbuf. The application doesn't need the NIC RX driver's
> > > >>>>> fast path to capture which date (or even which second) a packet
> > > >>>>> was received. Yes, it adds complexity to the application, but
> > we
> > > >>>>> can't set aside 64 bit for a generic timestamp. Or as a weird
> > tradeoff:
> > > >>>>> Put the fast moving 32 bit in the first cache line and the slow
> > > >>>>> moving 32 bit in the second cache line, as a placeholder for
> > the
> > > >>> application to fill out if needed.
> > > >>>>> Yes, it means that the application needs to check the time and
> > > >>>>> update its variable holding the slow moving time once every
> > > >>>>> second or so; but that should be doable without significant
> > effort.
> > > >>>>>>
> > > >>>>>> That's a good point, however without a 64 bit value, elapsed
> > > >>>>>> time between two arbitrary mbufs cannot be measured reliably
> > > >>>>>> due to
> > > >>> not
> > > >>>>>> enough context, one way or another the low resolution value is
> > > >>>>>> also
> > > >>>>> needed.
> > > >>>>>>
> > > >>>>>> Obviously latency-sensitive applications are unlikely to
> > > >>>>>> perform lengthy buffering and require this but I'm not sure
> > > >>>>>> about all the possible use-cases. Considering many NICs expose
> > > >>>>>> 64 bit
> > > >>> timestaps,
> > > >>>>>> I suggest we do not truncate them.
> > > >>>>>>
> > > >>>>>> I'm not a fan of the weird tradeoff either, PMDs will be
> > > >>>>>> tempted to fill the extra 32 bits whenever they can and negate
> > > >>>>>> the performance improvement of the first cache line.
> > > >>>>>
> > > >>>>> I would tend to agree, and I don't really see any convenient
> > way
> > > >>>>> to avoid putting in a 64-bit field for the timestamp in cache-
> > line 0.
> > > >>>>> If we are ok with having this overlap/partially overlap with
> > > >>>>> sequence number, it will use up an extra 4B of storage in that
> > > >>> cacheline.
> > > >>>>
> > > >>>> I agree about the lack of convenience! And Adrien certainly has
> > a
> > > >>> point about PMD temptations.
> > > >>>>
> > > >>>> However, I still don't think that a NICs ability to date-stamp a
> > > >>> packet is sufficient reason to put a date-stamp in cache line 0
> > of
> > > >>> the mbuf. Storing only the fast moving 32 bit in cache line 0
> > > >>> seems like a good compromise to me.
> > > >>>>
> > > >>>> Maybe you can find just one more byte, so it can hold 17 minutes
> > > >>>> with nanosecond resolution. (I'm joking!)
> > > >>>>
> > > >>>> Please don't sacrifice the sequence number for the
> > > >>>> seconds/hours/days
> > > >>> part a timestamp. Maybe it could be configurable to use a 32 bit
> > > >>> or
> > > >>> 64 bit timestamp.
> > > >>>>
> > > >>> Do you see both timestamp and sequence numbers being used
> > together?
> > > >>> I would have thought that apps would either use one or the other?
> > > >>> However, your suggestion is workable in any case, to allow the
> > > >>> sequence number to overlap just the high 32 bits of the
> > timestamp,
> > > >>> rather than the low.
> > > >>
> > > >> In our case, I can foresee sequence numbers used for packet
> > > >> processing and
> > > timestamps for timing analysis (and possibly for packet capturing,
> > > when being used). For timing analysis, we don’t need long durations,
> > > e.g. 4 seconds with 32 bit nanosecond resolution suffices. And for
> > > packet capturing we are perfectly capable of adding the slowly moving
> > > 32 bit of the timestamp to our output data stream without fetching it
> > from the mbuf.
> > > >>
> > >
> > > We should keep in mind that today we have the seqn field but it is
> > not
> > > used by any PMD. In case it is implemented, would it be a per-queue
> > sequence number?
> > > Is it useful from an application view?
> > >
> > > This field is only used by the librte_reorder library, and in my
> > > opinion, we should consider moving it in the second cache line since
> > it is not filled by the PMD.
> > >
> > >
> > > > For the 32-bit timestamp case, it might be useful to have a
> > > > right-shift value passed in to the ethdev driver. If we assume a
> > NIC
> > > > with nanosecond resolution, (or TSC value with resolution of that
> > > > order of magnitude), then the app can choose to have 1 ns
> > resolution
> > > > with 4 second wraparound, or alternatively 4ns resolution with 16
> > > > second wraparound, or even microsecond resolution with wrap around
> > > > of over
> > > an hour.
> > > > The cost is obviously just a shift op in the driver code per packet
> > > > - hopefully with multiple packets done at a time using vector
> > operations.
> > >
> > >
> > > About the timestamp, we can manage to find 64 bits in the first cache
> > > line, without sacrifying any field we have today. The question is
> > more
> > > for the fields we may want to add later.
> > >
> > > To answer to the question of the size of the timestamp, the first
> > > question is to know what is the precision required for the
> > applications using it?
> > 
> > As part of the 17.02 latency calculation feature, the requirement is to
> > report latency in nanoseconds. So, +1 on keeping timestamp as 64bits.
> > Since the feature is planned for 17.02, can we finalize on the
> > timestamp position in the mbuf struct?
> > Based on the decision, I am planning to make the change and send ABI
> > notice if needed.
> > 
> > Reshma
> > 
> > >
> > > I don't quite like the idea of splitting the timestamp in the 2 cache
> > > lines, I think it would not be easy to use.
> > >
> > >
> > > Olivier
> 
> Nanosecond precision in latency calculations does not require more than 32 bit, unless we must also be able to measure more than 4 seconds of latency. And please consider how many packets we would need to hold in memory to keep track of 4 seconds of traffic on a 100 Gbit/s link (if we are measuring network latency): With an average packet size of 300 B that would be 40 million packets.

Consider another, perhaps more realistic use-case: keeping a few relevant
packets from a stream for later analysis, which could occur outside of the
normal TX processing path for performance reasons. In this case 4 seconds
worth of context may not be enough to avoid losing information.

> If I recall correctly, the 17.02 feature is about measuring application latency, not network latency (as my calculation above is about). I don't consider application latency measuring a common feature for most applications, except for application debugging/profiling purposes (not network debugging purposes). So do we really need both nanosecond precision and the ability to measure 4+ seconds of latency?

Existing HW can already provide 64-bit precision, why truncate part of it?
The only valid reason would be that there is not enough room in the mbuf
header, which is not the case. Looks like today there is even enough room in
the first 64 bytes.

As suggested by Bruce, we can even union (part of) this field with another
if necessary, as long as we make sure both cannot be requested at the same
time (determining which field should be sacrificed is going to be tricky
though).

> Furthermore, if the timestamp isn't set by hardware, why not just take the entire array of packets pulled out from the NIC and set their timestamp to the same value, instead of setting them one by one in the PMD - that would also consolidate the expense of obtaining the time from some clock source.

Like any offload, some PMD processing is expected to convert the possibly
HW-specific value to a common mbuf format, however if hardware does not
report any kind of information relevant to timestamp individual packets, I
do not think the PMD should expose that feature and implement it in
software. It would be much worse, certainly not what the application
wanted as it could have done the same on its own.

What matters is to know when NIC receives a packet, not when the application
fetches it. You should assume there is a significant delay between these two
events, hence the need for it to be offloaded.

-- 
Adrien Mazarguil
6WIND