[dpdk-dev] [PATCH v12 0/6] add Tx preparation

Adrien Mazarguil adrien.mazarguil at 6wind.com
Mon Dec 5 19:10:21 CET 2016


On Mon, Dec 05, 2016 at 04:43:52PM +0000, Ananyev, Konstantin wrote:
[...]
> > On Fri, Dec 02, 2016 at 01:00:55AM +0000, Ananyev, Konstantin wrote:
> > [...]
> > > > On Wed, Nov 30, 2016 at 10:54:50AM +0000, Ananyev, Konstantin wrote:
> > > > [...]
> > > > > Do you have anything particular in mind here?
> > > >
> > > > Nothing in particular, so for the sake of the argument, let's suppose that I
> > > > would like to add a field to expose some limitation that only applies to my
> > > > PMD during TX but looks generic enough to make sense, e.g. maximum packet
> > > > size when VLAN tagging is requested.
> > >
> > > Hmm, I didn't hear about such limitations so far, but if it is real case -
> > > sure, feel free to submit the patch.
> > 
> > I won't, that was hypothetical.
> 
> Then why we discussing it? :)

Just to make a point, which is that new limitations may appear anytime and
tx_prepare() can now be used to check for them. First patch of the series
does it:

 +   uint16_t nb_seg_max;     /**< Max number of segments per whole packet. */
 +   uint16_t nb_mtu_seg_max; /**< Max number of segments per one MTU */

And states that:

 + * For each packet to send, the rte_eth_tx_prepare() function performs
 + * the following operations:
 + *
 + * - Check if packet meets devices requirements for tx offloads.
 + *
 + * - Check limitations about number of segments.
 + *
 + * - Check additional requirements when debug is enabled.
 + *
 + * - Update and/or reset required checksums when tx offload is set for packet.

It's like making this function mandatory IMO.

> > > > PMDs are free to set that field to some
> > > > special value (say, 0) if they do not care.
> > > >
> > > > Since that field exists however, conscious applications should check its
> > > > value for each packet that needs to be transmitted. This extra code causes a
> > > > slowdown just by sitting in the data path. Since it is not the only field in
> > > > that structure, the performance impact can be significant.
> > > >
> > > > Even though this code is inside applications, it remains unfair to PMDs for
> > > > which these tests are irrelevant. This problem is identified and addressed
> > > > by tx_prepare().
> > >
> > > I suppose the question is why do we need:
> > > uint16_t nb_seg_max;
> > > uint16_t nb_mtu_seg_max;
> > > as we now have tx_prepare(), right?
> > >
> > > For two reasons:
> > > 1. Some people might feel that tx_prepare() is not good (smart/fast) enough
> > > for them and would prefer to do necessary preparations for TX offloads themselves.
> > >
> > > 2. Even if people do use tx_prepare() they still should take this information into accout.
> > > As an example ixgbe can't TX packets with then 40 segments.
> > > Obviously ixbge_tx_prep() performs that check and returns an error.
> > 
> > Problem is that tx_prepare() also provides safeties which are not part of
> > tx_burst(), such as not going over nb_mtu_seg_max. Because of this and the
> > fact struct rte_eth_desc_lim can grow new fields anytime, application
> > developers will be tempted to just call tx_prepare() and focus on more
> > useful things.
> 
> NP with that, that was an intention beyond introducing it.
> 
> > Put another way, from a user's point of view, tx_prepare() is an opaque
> > function that greatly increases tx_burst()'s ability to send mbufs as
> > requested, with extra error checking on top; applications not written to run
> > on a specific PMD/device (all of them ideally) will thus call tx_prepare()
> > at some point.
> > 
> > > But it wouldn't try to merge/reallocate mbufs for you.
> > > User still has to do it himself, or just prevent creating such long chains somehow.
> > 
> > Yes, that's another debate. PMDs could still implement a software fallback
> > for unlikely slow events like these. The number of PMDs is not going to
> > decrease, each device having its own set of weird limitations in specific
> > cases, PMDs should do their best to process mbufs even if that means slowly
> > due to the lack of preparation.
> > 
> > tx_prepare() has its uses but should really be optional, in the sense that
> > if that function is not called, tx_burst() should deal with it somehow.
> 
> As I said before, I don't think it is a good idea to put everything in tx_burst().
> If PMD driver prefer things that way, yes tx_burst() can deal with each and
> possible offload requirement itself, but it shouldn't be mandatory. 

In effect, having to call tx_prepare() otherwise makes this step mandatory
anyway. Looks like we are not going to agree here.

> > > > Thanks to tx_prepare(), these checks are moved back into PMDs where they
> > > > belong. PMDs that do not need them do not have to provide support for
> > > > tx_prepare() and do not suffer any performance impact as result;
> > > > applications only have to make sure tx_prepare() is always called at some
> > > > point before tx_burst().
> > > >
> > > > Once you reach this stage, you've effectively made tx_prepare() mandatory
> > > > before tx_burst(). If some bug occurs, then perhaps you forgot to call
> > > > tx_prepare(), you just need to add it. The total cost for doing TX is
> > > > therefore tx_prepare() + tx_burst().
> > > >
> > > > I'm perhaps a bit pessimistic mind you, but I do not think tx_prepare() will
> > > > remain optional for long. Sure, PMDs that do not implement it do not care,
> > > > I'm focusing on applications, for which the performance impact of calling
> > > > tx_prepare() followed by tx_burst() is higher than a single tx_burst()
> > > > performing all the necessary preparation at once.
> > > >
> > > > [...]
> > > > > > Following the same logic, why can't such a thing be made part of the TX
> > > > > > burst function as well (through a direct call to rte_phdr_cksum_fix()
> > > > > > whenever necessary). From an application standpoint, what are the advantages
> > > > > > of having to:
> > > > > >
> > > > > >  if (tx_prep()) // iterate and update mbufs as needed
> > > > > >      tx_burst(); // iterate and send
> > > > > >
> > > > > > Compared to:
> > > > > >
> > > > > >  tx_burst(); // iterate, update as needed and send
> > > > >
> > > > > I think that was discussed extensively quite a lot previously here:
> > > > > As Thomas already replied - main motivation is to allow user
> > > > > to execute them on different stages of packet TX pipeline,
> > > > > and probably on different cores.
> > > > > I think that provides better flexibility to the user to when/where
> > > > > do these preparations and hopefully would lead to better performance.
> > > >
> > > > And I agree, I think this use case is valid but does not warrant such a high
> > > > penalty when your application does not need that much flexibility. Simple
> > > > (yet conscious) applications need the highest performance. Complex ones as
> > > > you described already suffer quite a bit from IPCs and won't mind a couple
> > > > of extra CPU cycles right?
> > >
> > > It would mean an extra cache-miss for every packet, so I think performance hit
> > > would be quite significant.
> > 
> > A performance hit has to occur somewhere regardless, because something has
> > to be done in order to send packets that need it. Whether this cost is in
> > application code or in a PMD function, it remains part of TX.
> 
> Depending on the place the final cost would differ quite a lot.
> If you call tx_prepare() somewhere close to the place where you fill the packet header
> contents, then most likely the data that tx_prepare() has to access will be already in the cache.
> So the performance penalty will be minimal.
> If you'll try to access the same data later (at tx_burst), then the possibility that it would still
> be in cache is much less.
> If you calling tx_burst() from other core then data would for sure be out of cache,
> and  even worse can still be in another core cache.

Well sure, that's why I also think tx_prepare() has its uses, only that
since tx_prepare() is optional, tx_burst() should provide the same
functionality when tx_prepare() is not called.

> > > About the 'simple' case when tx_prep() and tx_burst() are called on the same core,
> > > Why do you believe that:
> > > tx_prep(); tx_burst(); would be much slower than tx_burst() {tx_prep(), ...}?
> > 
> > I mean instead of two function calls with their own loops:
> > 
> >  tx_prepare() { foreach (pkt) { check(); extra_check(); ... } }
> > 
> >  tx_burst() { foreach (pkt) { check(); stuff(); ... } }
> > 
> > You end up with one:
> > 
> >  tx_burst() { foreach (pkt) { check(); extra_check(); stuff(); ... } }
> > 
> > Which usually is more efficient.
> 
> I really doubt that.
> If it would be that, what is the point to process packet in bulks?
> Usually dividing processing into different stages and at each stage processing
> multiple packet at once helps to improve performance.
> At  least for IA.
> Look for example how we had to change l3fwd to improve its performance.

Depends quite a bit on usage pattern. It is less efficient for applications
that do not modify mbuf contents because of the additional function call and
inner loop.

Note that I'm only pushing for the ability to conveniently address both
cases with maximum performance.

> > > tx_prep() itself is quite expensive, let say for Intel HW it includes:
> > > - read mbuf fileds (2 cache-lines),
> > > - read packet header (1/2 cache-lines)
> > > - calculate pseudo-header csum
> > >  - update packet header
> > > Comparing to that price of extra function call seems neglectable
> > > (if we TX packets in bursts of course).
> > 
> > We agree its performance is a critical issue then, sharing half the read
> > steps with tx_burst() would make sense to me.
> 
> I didn't understand that sentence.

I meant this step can be shared (in addition to loop etc):

 - read mbuf fileds (2 cache-lines),

> > > > Yes they will, therefore we need a method that satisfies both cases.
> > > >
> > > > As a possible solution, a special mbuf flag could be added to each mbuf
> > > > having gone through tx_prepare(). That way, tx_burst() could skip some
> > > > checks and things it would otherwise have done.
> > >
> > > That's an interesting idea, but it has one drawback:
> > > As I understand, it means that from now on if user doing preparations on his own,
> > > he had to setup this flag, otherwise tx_burst() would do extra unnecessary work.
> > > So any existing applications that using TX offloads and do preparation by themselves
> > > would have to be modified to avoid performance loss.
> > 
> > In my opinion, users should not do preparation on their own.
> 
> People already do it now.

But we do not want them to anymore thanks to this new API, for reasons
described in the motivation section of the cover letter, right?

> > If we provide a
> > generic method, it has to be fast enough to replace theirs. Perhaps not as
> > fast since it would work with all PMDs (usual trade-off), but acceptably so.
> > 
> > > > Another possibility, telling the PMD first that you always intend to use
> > > > tx_prepare() and getting a simpler/faster tx_burst() callback as a result.
> > >
> > > That what we have right now (at least for Intel HW):
> > > it is a user responsibility to do the necessary preparations/checks before calling tx_burst().
> > > With tx_prepare() we just remove from user the headache to implement tx_prepare() on his own.
> > > Now he can use a 'proper' PMD provided function.
> > >
> > > My vote still would be for that model.
> > 
> > OK, then in a nutshell:
> > 
> > 1. Users are not expected to perform preparation/checks themselves anymore,
> >    if they do, it's their problem.
> 
> I think we need to be backward compatible here.
> If the existing app doing what tx_prepare() supposed to do, it should keep working.

It should work, only if they keep doing it as well as call tx_burst()
directly, they will likely get lower performance.

> > 2. If configured through an API to be defined, tx_burst() can be split in
> >    two and applications must call tx_prepare() at some point before
> >    tx_burst().
> > 
> > 3. Otherwise tx_burst() should perform the necessary preparation and checks
> >    on its own by default (when tx_prepare() is not expected).
> 
> As I said before, I don't think it should be mandatory for tx_burst() to do what tx_prepare() does.
> If some particular implementation of tx_burst() prefers to do things that way - that's fine.
> But it shouldn't be required to.

You're right, however applications might find it convenient. I think most
will end up with something like the following:

 if (tx_prepare(pkts))
     tx_burst(pkts));

> > 4. We probably still need some mbuf flag to mark mbufs that cannot be
> >    modified, the refcount could also serve as a hint.
> 
> If mbuf can't be modified, you probably just wouldn't call the function that supposed to do that,
> tx_prepare() in that case.  

I think it would be easier to document what offload flags may cause the
tx_burst() function to modify mbuf contents, so applications have the
ability to set or strip these flags on a mbuf basis. That way there is no
need to call tx_prepare() without knowing exactly what it's going to do.

-- 
Adrien Mazarguil
6WIND


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