Re: [dpdk-dev] [PATCH v2 5/6] rte_sched: allow reading statistics without clearing

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 16 Mar 2015 19:58:51 +0000
"Dumitrescu, Cristian" <cristian.dumitrescu@intel.com> wrote:

> 
> 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > Sent: Thursday, March 12, 2015 11:06 PM
> > To: Dumitrescu, Cristian
> > Cc: dev@dpdk.org; Stephen Hemminger
> > Subject: Re: [PATCH v2 5/6] rte_sched: allow reading statistics without
> > clearing
> > 
> > On Thu, 12 Mar 2015 19:28:11 +0000
> > "Dumitrescu, Cristian" <cristian.dumitrescu@intel.com> wrote:
> > 
> > > Hi Stephen,
> > >
> > > Thank you for adding flexibility over clearing the stats or not.
> > >
> > > I have one concern though: why change the stats read API to add a clear
> > parameter rather than keep prototype for the stats functions unchanged and
> > add the flag as part of the port creation parameters in struct
> > rte_sched_port_params? This parameter could be saved into the internal
> > struct rte_sched_port, which is passed (as a pointer) to the stats read
> > functions. In my opinion, this approach is slightly more elegant and it keeps
> > the changes to a minimum.
> > >
> > > int
> > > rte_sched_queue_read_stats(struct rte_sched_port *port,
> > > 	uint32_t queue_id,
> > > 	struct rte_sched_queue_stats *stats,
> > > 	uint16_t *qlen)
> > > {
> > > 	...
> > > 	if (port->clear_stats_on_read)
> > > 		memset(...);
> > > }
> > >
> > > I made this suggestion during the previous round, but I did not get any
> > opinion from you on it yet.
> > >
> > > Regards,
> > > Cristian
> > 
> > I rejected the config parameter idea because I don't like it is inconsistent
> > with other statistics in DPDK and in related software. There is not a
> > config parameter that changes what BSD or Linux kernel API does.
> 
> Your approach has the advantage of being able to clear/not clear the stats per each read operation rather than configuring the behavior globally. I think this approach allows for the ultimate flexibility, so I am OK to go with it.
> 
> > 
> > The only reason for keeping the read and clear in one operation is
> > because you like it, and there are somebody might have built code
> > that expects it.
> >
> 
> Clearing the stats with a delay after the stats have been read is prone to a race condition, as during this time more packets could be processed, and these packets will not show up in the counters that the user read.
> I think it depends on the need of each particular application whether this race condition is important or not: if the counters are read rarely (e.g. once per day) and only course accuracy is required, the error is probably irrelevant; if the app is looking for fine great accuracy (e.g. rate measurement, debugging, etc), then the error is not allowed. You seem to favour the former and ignore the later case.
> 
> 
> > Changing the function signature is a nice red flag so that people will
> > notice at change.
> 
> There is a small API change here. I am OK with it for the above reasons, provided that there are no objections on this from other contributors.
> 

If you really wanted a fast/safe read and clear operation, how about using
exchange instruction to exchange in a zero?