On Tue, Feb 17, 2015 at 10:49:24AM -0500, Neil Horman wrote: > On Tue, Feb 17, 2015 at 01:50:58PM +0000, Bruce Richardson wrote: > > On Tue, Feb 17, 2015 at 02:28:02PM +0100, Olivier MATZ wrote: > > > Hi Bruce, > > > > > > On 02/17/2015 01:25 PM, Bruce Richardson wrote: > > > >On Mon, Feb 16, 2015 at 06:34:37PM +0100, Thomas Monjalon wrote: > > > >>2015-02-16 15:16, Bruce Richardson: > > > >>>In this specific instance, given that the application does little > > > >>>else, there > > > >>>is no real advantage to using the callbacks - it's just to have a > > > >>>simple example > > > >>>of how they can be used. > > > >>> > > > >>>Where callbacks are really designed to be useful, is for extending or > > > >>>augmenting > > > >>>hardware capabilities. Taking the example of sequence numbers - to use > > > >>>the most > > > >>>trivial example - an application could be written to take advantage of > > > >>>sequence > > > >>>numbers written to packets by the hardware which received them. > > > >>>However, if such > > > >>>an application was to be used with a NIC which does not provide > > > >>>sequence numbering > > > >>>capability, for example, anything using ixgbe driver, the application > > > >>>writer has > > > >>>two choices - either modify his application code to check each packet > > > >>>for > > > >>>a sequence number in the data path, and add it there post-rx, or > > > >>>alternatively, > > > >>>to check the NIC capabilities at initialization time, and add a > > > >>>callback there > > > >>>at initialization, if the hardware does not support it. In the latter > > > >>>case, > > > >>>the main packet processing body of the application can be written as > > > >>>though > > > >>>hardware always has sequence numbering capability, safe in the > > > >>>knowledge that > > > >>>any hardware not supporting it will be back-filled by a software > > > >>>fallback at > > > >>>initialization-time. > > > >>> > > > >>>By the same token, we could also look to extend hardware capabilities. > > > >>>For > > > >>>different filtering or hashing capabilities, there can be limits in > > > >>>hardware > > > >>>which are far less than what we need to use in software. Again, > > > >>>callbacks will > > > >>>allow the data path to be written in a way that is oblivious to the > > > >>>underlying > > > >>>hardware limits, because software will transparently fill in the gaps. > > > >>> > > > >>>Hope this makes the use case clear. > > > >> > > > >>After thinking more about these callbacks, I realize these callbacks > > > >>won't > > > >>help, as Olivier said. > > > >> > > > >>With callback, > > > >>1/ application checks device capability > > > >>2/ application provides hardware emulation as DPDK callback > > > >>3/ application forgets previous steps > > > >>4/ application calls DPDK Rx > > > >>5/ DPDK calls callback (without calling optimization) > > > >> > > > >>Without callback, > > > >>1/ application checks device capability > > > >>2/ application provides hardware emulation as internal function > > > >>3/ application set an internal device-flag to enable this function > > > >>4/ application calls DPDK Rx > > > >>5/ application calls the hardware emulation if flag is set > > > >> > > > >>So the only difference is to keep persistent the device information in > > > >>the application instead of storing it as a function pointer in the > > > >>DPDK struct. > > > >>You can also be faster with this approach: at initialization time, > > > >>you can check that your NIC supports the feature and use a specific > > > >>mainloop that adds or not the sequence number without any runtime > > > >>test. > > > > > > > >That is assuming that all NICs are equal on your system. It's also > > > >assuming > > > >that you only have a single point in your application where you call RX > > > >or > > > >TX burst. In the case where you have a couple of different NICs on the > > > >system, > > > >or where you want to write an application to take advantage of > > > >capabilities of > > > >different NICs, the ability to resolve all these difference at > > > >initialization > > > >time is useful. The main packet handling code can be written with just > > > >the > > > >processing of packets in mind, rather than having to have a set of > > > >branches > > > >after each RX burst call, or before each TX burst call, to "smooth out" > > > >the > > > >different NIC capabilities. > > > > > > > >As for the option of maintaining different main loops for different NICs > > > >with > > > >different capabilities - that sounds like a maintenance nightmare to > > > >me, due to duplicated code! Callbacks is a far cleaner solution than > > > >that IMHO. > > > > > > Why not just provide a function like this: > > > > > > rte_do_unsupported_stuff_by_software(m[], m_count, wanted_features, > > > dev_feature_flags) > > > > > > This function can be called (or not) from the application mainloop. > > > You don't need to maintain several mainloops (for each device) as > > > the specific work will be done depending on the given flags. And the > > > applications that do not require these features (most applications?) > > > are not penalized at all. > > > > Have you measured the performance hit due to this proposed change? In my > > tests > > it's very, very small, even for the fastest vectorized path. If performance > > is > > a real concern, I'm happy enough to have this as a compile-time option so > > that > > those who can't take the small performance hit can avoid it. > > > How can you assert performance metrics on a patch like this? The point of the > change is to allow a callback to an application defined function, the contents > of which are effectively arbitrary. Not saying that its the wrong thing to > do, > but you can't really claim performance is not impacted, because the details of > whats executed is outside your purview. > Neil > I think the performance hit being referenced is a hit due to the patch itself without any callbacks being in use. (That was certainly my assumption in replying)
/Bruce
