Re: [dpdk-dev] [PATCH v5 0/3] new headroom stats library and example application

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2015-02-23 15:55, Jastrzebski, MichalX K:
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > 2015-02-23 14:36, Jastrzebski, MichalX K:
> > > From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > > > 2015-02-20 15:46, Jastrzebski, MichalX K:
> > > > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Neil Horman
> > > > > > On Thu, Feb 19, 2015 at 01:18:41PM +0100, Pawel Wodkowski wrote:
> > > > > > > Hi community,
> > > > > > > I would like to introduce library for measuring load of some arbitrary
> > > > jobs.
> > > > > > > It can be used to profile every kind of job sets on any arbitrary
> > execution
> > > > unit
> > > > > > > or tasking library.
> > > > > > >
> > > > > > > In provided l2fwd-headroom example I demonstrate how to use this
> > > > library to
> > > > > > > select optimal rx burst poll time. Jobs are selected by using existing
> > > > rte_timer
> > > > > > > library calls. This example does no limit possible schemes on which
> > this
> > > > > > > library can be used.
> > > > > > >
> > > > > > > Pawel Wodkowski (3):
> > > > > > >   librte_headroom: New library for checking core/system/app load
> > > > > > >   examples: introduce new l2fwd-headroom example
> > > > > > >   MAINTAINERS: claim responsibility for headroom library and
> > example
> > > > app
> > > > > >
> > > > > > I'm sorry but I still fail to see how this is a particularly useful library.  It
> > > > > > clearly works fine, but it composes an application event loop in its
> > own
> > > > > > terms,
> > > > > > and measures stats based on that.  While thats ok, any application is
> > > > already
> > > > > > going to have to write its own event loop, and can makethe same
> > > > > > measurements
> > > > > > synchnously within that loop, using alot less code to optimize its
> > polling
> > > > time.
> > > > > >
> > > > > > In other words, I think this is one of those cases where this library is
> > > > > > probably somewhat useful for anyone who just wants to write an
> > > > application
> > > > > > in
> > > > > > terms the semantics exposed by this library, but not at all useful for
> > > > anyone
> > > > > > else.  I'd personally rather not have the extra code to maintain here.
> > > > > >
> > > > > > Stephen just gave a presentation at netdev about some of the
> > > > performance
> > > > > > optimization measurements Brocade did with DPDK and how they fine
> > > > tuned
> > > > > > their
> > > > > > environment.  One of the big take aways for me was that making time
> > > > based
> > > > > > measurements (especially if it was using the tsc), created cpu stalls
> > that
> > > > > > skewed the measurements, and so the best optimizations they made
> > > > avoided
> > > > > > time
> > > > > > measurements, opting instead for packet count metrics.
> > > > > >
> > > > > > Neil
> > > > >
> > > > > Hi Neil,
> > > > >
> > > > > I think this library offers something quite useful probably not for
> > everyone,
> > > > > but for many people that use DPDK, and it is measuring quite
> > accurately,
> > > > > how many spare cycles a CPU have after executing any serial tasks (as
> > you
> > > > will know).
> > > > > If you look at two places in example application: main_loop()
> > > > > and l2fwd_fwd() functions, you will see two possible approach there,
> > but
> > > > > this is not limited to that. You can even nest headroom objects and
> > > > measure
> > > > > process time of particular packets type.
> > > > > Of course, this will add an overhead due to the measurements,
> > > > > but that time is also measured, so any user can know what is the
> > relative
> > > > > time "wasted" for measuring all this.
> > > > > If time delays are measured in bigger timestamps, are handled reliably,
> > > > > the cost of measuring will be low.
> > > > > I find this quite similar to the power library case. I would say that library
> > is
> > > > not useful
> > > > > for every application, but there are several cases where it can be
> > > > > (as demonstrated with l3fwd-power app).
> > > > >
> > > > > About your last bit, not sure if I understood it right, but in case of the
> > > > included sample app,
> > > > > the main measurement to see if we are overusing a CPU is the packet
> > count
> > > > > in a queue (in this case RX queue), and I believe this should be used for
> > > > other apps,
> > > > > especially in those that use a pipeline model, where queues and rings
> > are
> > > > the key part.
> > > > >
> > > > > As a final point, last week (12th of February), there was a request for a
> > > > tool/library like this
> > > > > from a user in the mailing list (Ilan Borenshtein), which indicates that
> > this
> > > > would be useful
> > > > > (probably not just for him, but for others). It probably could be achieved
> > by
> > > > the user
> > > > > by adding their own code, but I believe this library would be a good-to-
> > > > have,
> > > > > in case a user is looking for an easy way to calculate the exposed above.
> > > > > Let us give the users an example of this method and we will expand it
> > with
> > > > more
> > > > > advanced application that may show capabilities of dynamic load
> > scaling
> > > > based on headroom library measurement.
> > > >
> > > Hi Thomas,
> > >
> > > > I wonder how this library is related to DPDK.
> > > > I'm not against its integration, though the question must be asked.
> > > > DPDK is a set of libraries. What kind of library fit with DPDK goals and
> > > > deserve to be integrated?
> > > >
> > > I think this library fits into dpdk goals, because it is simple and optimized
> > for fast packet processing.
> > 
> > I don't have a strong opinion here. If anyone else wants to comment, please
> > speak now.
> > 
> > > The library provides an easy way for existing DPDK users to modify their
> > applications to measure available CPU headroom.
> > > If we integrate it, users will verify it and decide what else they need and we
> > will implement this.
> > 
> > Do you mean that you plan to add some features to this library?
> > Is it going to stay at providing some stats or could you make some actions
> > like time-sharing helpers?
> What do you mean here saying time-sharing? 

I mean helpers to stop processing at a defined rate in order to share CPU. 

> > > > I don't know whether it's related but nobody acknowledged this patchset.
> > > We were waiting for Neil's final comments. He did not mention anything
> > else since the last time.
> > > When Pawel sends the next version with the copyright date corrected,
> > Pablo will ack this.
> > >
> > > > I also feel that the name of this library is a bit too vague. Some people
> > > > were asking first what means "headroom". It's actually for CPU headroom
> > > > monitoring.
> > > > What about "cpuheadroom", "cpuheadroomstat", "jobstat"?
> > >
> > > I think we can change the name to "cpuheadroom" as it describes more
> > clear this library.
> > 
> > If you're focusing on CPU usage with possible actions, yes.
> > If you're focusing on decision helper, jobstat would be better IMHO.
> > 
> > > > Last comment, less important: as many of your colleagues, you don't pay
> > > > attention to the copyright dates. I'm pretty sure this code was not written
> > > > in 2010. So why claiming it?
> > > Sorry, we will fix this of course and pay more attention now.
> > 
> > OK, thanks