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

Dumitrescu, Cristian cristian.dumitrescu at intel.com
Wed Dec 2 17:48:17 CET 2015



> -----Original Message-----
> From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> Sent: Sunday, November 29, 2015 8:47 PM
> To: Dumitrescu, Cristian <cristian.dumitrescu at intel.com>
> Cc: dev at dpdk.org; Stephen Hemminger <stephen at 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 at 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?

> +
>  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?



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