Re: [dpdk-dev] [PATCH 3/3] rte_sched: eliminate floating point in calculating byte clock

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 2 Dec 2015 16:48:17 +0000
"Dumitrescu, Cristian" <cristian.dumitrescu@intel.com> wrote:

> 
> 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > Sent: Sunday, November 29, 2015 8:47 PM
> > To: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>
> > Cc: dev@dpdk.org; Stephen Hemminger <stephen@networkplumber.org>
> > Subject: [PATCH 3/3] rte_sched: eliminate floating point in calculating byte
> > clock
> > 
> > The old code was doing a floating point divide for each rte_dequeue()
> > which is very expensive. Change to using fixed point scaled inverse
> > multiply. To maintain equivalent precision, scaled math is used.
> > The application ABI is the same.
> > 
> > This improved performance from 5Gbit/sec to 10 Gbit/sec when configured
> > for 10 Gbit/sec rate.
> > 
> > There was some feedback from Cristian that he wanted a better
> > solution and was going to give one, but none was provided.
> > For 2.2 this is a better solution than existing code, if someone
> > has a better version I would love to see it.
> > 
> > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> > ---
> >  lib/librte_sched/rte_sched.c | 23 ++++++++++++++++++-----
> >  1 file changed, 18 insertions(+), 5 deletions(-)
> > 
> > diff --git a/lib/librte_sched/rte_sched.c b/lib/librte_sched/rte_sched.c
> > index 16acd6b..cfae136 100644
> > --- a/lib/librte_sched/rte_sched.c
> > +++ b/lib/librte_sched/rte_sched.c
> > @@ -47,6 +47,7 @@
> >  #include "rte_bitmap.h"
> >  #include "rte_sched_common.h"
> >  #include "rte_approx.h"
> > +#include "rte_reciprocal.h"
> > 
> >  #ifdef __INTEL_COMPILER
> >  #pragma warning(disable:2259) /* conversion may lose significant bits */
> > @@ -62,6 +63,11 @@
> >  #define RTE_SCHED_PIPE_INVALID                UINT32_MAX
> >  #define RTE_SCHED_BMP_POS_INVALID             UINT32_MAX
> > 
> > +/* Scaling for cycles_per_byte calculation
> > + * Chosen so that minimum rate is 480 bit/sec
> > + */
> > +#define RTE_SCHED_TIME_SHIFT		      8
> 
> Stephen, can you please elaborate why we need to shift the dividend at all and why the shift value was picked as 8? Is 8 a hard constraint? How does this affect the scheduling precision/accuracy?

The shift value is a tradeoff for scaled math. The bigger the shift
the finer the resolution, but at the risk of overflow in the cycles_per_byte.
The value was chosen as a tradeoff based on current CPU clock rate (TSC)
and minimum rate.

> > +
> >  struct rte_sched_subport {
> >  	/* Token bucket (TB) */
> >  	uint64_t tb_time; /* time of last update */
> > @@ -215,7 +221,7 @@ struct rte_sched_port {
> >  	uint64_t time_cpu_cycles;     /* Current CPU time measured in CPU
> > cyles */
> >  	uint64_t time_cpu_bytes;      /* Current CPU time measured in bytes
> > */
> >  	uint64_t time;                /* Current NIC TX time measured in bytes */
> > -	double cycles_per_byte;       /* CPU cycles per byte */
> > +	struct rte_reciprocal inv_cycles_per_byte; /* CPU cycles per byte */
> > 
> >  	/* Scheduling loop detection */
> >  	uint32_t pipe_loop;
> > @@ -610,7 +616,7 @@ struct rte_sched_port *
> >  rte_sched_port_config(struct rte_sched_port_params *params)
> >  {
> >  	struct rte_sched_port *port = NULL;
> > -	uint32_t mem_size, bmp_mem_size, n_queues_per_port, i;
> > +	uint32_t mem_size, bmp_mem_size, n_queues_per_port, i,
> > cycles_per_byte;
> > 
> >  	/* Check user parameters. Determine the amount of memory to
> > allocate */
> >  	mem_size = rte_sched_port_get_memory_footprint(params);
> > @@ -661,7 +667,10 @@ rte_sched_port_config(struct
> > rte_sched_port_params *params)
> >  	port->time_cpu_cycles = rte_get_tsc_cycles();
> >  	port->time_cpu_bytes = 0;
> >  	port->time = 0;
> > -	port->cycles_per_byte = ((double) rte_get_tsc_hz()) / ((double)
> > params->rate);
> > +
> > +	cycles_per_byte = (rte_get_tsc_hz() << RTE_SCHED_TIME_SHIFT)
> > +		/ params->rate;
> > +	port->inv_cycles_per_byte = rte_reciprocal_value(cycles_per_byte);
> > 
> >  	/* Scheduling loop detection */
> >  	port->pipe_loop = RTE_SCHED_PIPE_INVALID;
> > @@ -2088,11 +2097,15 @@ 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;
> > -	double bytes_diff = ((double) cycles_diff) / port->cycles_per_byte;
> > +	uint64_t bytes_diff;
> > +
> > +	/* 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_bytes += (uint64_t) bytes_diff;
> > +	port->time_cpu_bytes += bytes_diff;
> >  	if (port->time < port->time_cpu_bytes)
> >  		port->time = port->time_cpu_bytes;
> > 
> > --
> > 2.1.4
> 
> Can you provide some insight into how you tested this code and the test vectors you used?

We tested with 10 gbit link and range of rates from 10k bit up to 10 gbit.