DPDK patches and discussions
 help / color / mirror / Atom feed
From: "Singh, Jasvinder" <jasvinder.singh@intel.com>
To: "dev@dpdk.org" <dev@dpdk.org>, Alan Dewar <alan.dewar@att.com>
Cc: "alangordondewar@gmail.com" <alangordondewar@gmail.com>,
	Thomas Monjalon <thomas@monjalon.net>,
	"Dumitrescu, Cristian" <cristian.dumitrescu@intel.com>
Subject: Re: [dpdk-dev] [PATCH v2] sched: fix port time rounding error
Date: Mon, 6 Jul 2020 21:20:48 +0000	[thread overview]
Message-ID: <CY4PR11MB0072C105F5E0AFEA0F8BFC5AE0690@CY4PR11MB0072.namprd11.prod.outlook.com> (raw)
In-Reply-To: <1765563.QPqcEuCtyy@thomas>



> -----Original Message-----
> From: Thomas Monjalon <thomas@monjalon.net>
> Sent: Sunday, July 5, 2020 9:42 PM
> To: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>; Singh, Jasvinder
> <jasvinder.singh@intel.com>
> Cc: dev@dpdk.org; Alan Dewar <alan.dewar@att.com>;
> alangordondewar@gmail.com
> Subject: Re: [dpdk-dev] [PATCH v2] sched: fix port time rounding error
> 
> Cristian, Jasvinder, please review.
> 
> 25/06/2020 11:59, alangordondewar@gmail.com:
> > From: Alan Dewar <alan.dewar@att.com>
> >
> > The QoS scheduler works off port time that is computed from the number
> > of CPU cycles that have elapsed since the last time the port was
> > polled.   It divides the number of elapsed cycles to calculate how
> > many bytes can be sent, however this division can generate rounding
> > errors, where some fraction of a byte sent may be lost.
> >
> > Lose enough of these fractional bytes and the QoS scheduler
> > underperforms.  The problem is worse with low bandwidths.
> >
> > To compensate for this rounding error this fix doesn't advance the
> > port's time_cpu_cycles by the number of cycles that have elapsed, but
> > by multiplying the computed number of bytes that can be sent (which
> > has been rounded down) by number of cycles per byte.
> > This will mean that port's time_cpu_cycles will lag behind the CPU
> > cycles momentarily.  At the next poll, the lag will be taken into
> > account.
> >
> > v2:
> > If the cycles value wraps (100 year+) reset the port's cpu cycle back
> > to zero.
> >
> > Fixes: de3cfa2c98 ("sched: initial import")
> >
> > Signed-off-by: Alan Dewar <alan.dewar@att.com>
> > ---
> >  lib/librte_sched/rte_sched.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/librte_sched/rte_sched.c
> > b/lib/librte_sched/rte_sched.c index c0983ddda..7c022cd61 100644
> > --- a/lib/librte_sched/rte_sched.c
> > +++ b/lib/librte_sched/rte_sched.c
> > @@ -222,6 +222,7 @@ struct rte_sched_port {
> >  	uint64_t time_cpu_bytes;      /* Current CPU time measured in bytes
> */
> >  	uint64_t time;                /* Current NIC TX time measured in bytes */
> >  	struct rte_reciprocal inv_cycles_per_byte; /* CPU cycles per byte */
> > +	uint64_t cycles_per_byte;
> >
> >  	/* Grinders */
> >  	struct rte_mbuf **pkts_out;
> > @@ -852,6 +853,7 @@ rte_sched_port_config(struct
> rte_sched_port_params *params)
> >  	cycles_per_byte = (rte_get_tsc_hz() << RTE_SCHED_TIME_SHIFT)
> >  		/ params->rate;
> >  	port->inv_cycles_per_byte = rte_reciprocal_value(cycles_per_byte);
> > +	port->cycles_per_byte = cycles_per_byte;
> >
> >  	/* Grinders */
> >  	port->pkts_out = NULL;
> > @@ -2673,16 +2675,21 @@ static inline void
> > rte_sched_port_time_resync(struct rte_sched_port *port)  {
> >  	uint64_t cycles = rte_get_tsc_cycles();
> > -	uint64_t cycles_diff = cycles - port->time_cpu_cycles;
> > +	uint64_t cycles_diff;
> >  	uint64_t bytes_diff;
> >  	uint32_t i;
> >
> > +	if (cycles < port->time_cpu_cycles)
> > +		port->time_cpu_cycles = 0;
> > +
> > +	cycles_diff = cycles - port->time_cpu_cycles;
> >  	/* Compute elapsed time in bytes */
> >  	bytes_diff = rte_reciprocal_divide(cycles_diff <<
> RTE_SCHED_TIME_SHIFT,
> >  					   port->inv_cycles_per_byte);
> >
> >  	/* Advance port time */
> > -	port->time_cpu_cycles = cycles;
> > +	port->time_cpu_cycles +=
> > +		(bytes_diff * port->cycles_per_byte) >>
> RTE_SCHED_TIME_SHIFT;
> >  	port->time_cpu_bytes += bytes_diff;
> >  	if (port->time < port->time_cpu_bytes)
> >  		port->time = port->time_cpu_bytes;
> >
> 

Acked-by: Jasvinder Singh <jasvinder.singh@intel.com>


  reply	other threads:[~2020-07-06 21:20 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-16  8:48 [dpdk-dev] [PATCH] " alangordondewar
2020-04-17 21:19 ` Dumitrescu, Cristian
2020-04-20 11:23   ` Singh, Jasvinder
2020-04-21  8:21     ` Dewar, Alan
2020-06-24 22:50       ` Thomas Monjalon
2020-06-25  8:32         ` Singh, Jasvinder
2020-06-25  8:40           ` Alan Dewar
2020-06-25  9:59 ` [dpdk-dev] [PATCH v2] " alangordondewar
2020-07-05 20:41   ` Thomas Monjalon
2020-07-06 21:20     ` Singh, Jasvinder [this message]
2020-07-06 23:01       ` Thomas Monjalon
2020-08-20 14:32   ` Kevin Traynor
2020-08-21 15:28     ` Kinsella, Ray
2020-09-07 10:09       ` Kevin Traynor

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CY4PR11MB0072C105F5E0AFEA0F8BFC5AE0690@CY4PR11MB0072.namprd11.prod.outlook.com \
    --to=jasvinder.singh@intel.com \
    --cc=alan.dewar@att.com \
    --cc=alangordondewar@gmail.com \
    --cc=cristian.dumitrescu@intel.com \
    --cc=dev@dpdk.org \
    --cc=thomas@monjalon.net \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).