Ubuntu Manpage: ioctl_eventpoll, EPIOCSPARAMS, EPIOCGPARAMS - ioctl() operations for epoll file descriptors

name
library
synopsis
description
return value
errors
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examples
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NAME

       ioctl_eventpoll, EPIOCSPARAMS, EPIOCGPARAMS - ioctl() operations for epoll file descriptors

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/epoll.h>  /* Definition of EPIOC* constants */
       #include <sys/ioctl.h>

       int ioctl(int fd, EPIOCSPARAMS, const struct epoll_params *argp);
       int ioctl(int fd, EPIOCGPARAMS, struct epoll_params *argp);

       #include <sys/epoll.h>

       struct epoll_params {
           uint32_t  busy_poll_usecs;   /* Number of usecs to busy poll */
           uint16_t  busy_poll_budget;  /* Max packets per poll */
           uint8_t   prefer_busy_poll;  /* Boolean preference  */

           /* pad the struct to a multiple of 64bits */
           uint8_t   __pad;             /* Must be zero */
       };

DESCRIPTION

EPIOCSPARAMS
Set the epoll_params structure to configure the operation of epoll. Refer to the structure
description below to learn what configuration is supported.

EPIOCGPARAMS
Get the current epoll_params configuration settings.

All operations documented above must be performed on an epoll file descriptor, which can be obtained with
a call to epoll_create(2) or epoll_create1(2).

The epoll_params structure
argp.busy_poll_usecs denotes the number of microseconds that the network stack will busy poll. During
this time period, the network device will be polled repeatedly for packets. This value cannot exceed
INT_MAX.

argp.busy_poll_budget denotes the maximum number of packets that the network stack will retrieve on each
poll attempt. This value cannot exceed NAPI_POLL_WEIGHT (which is 64 as of Linux 6.9), unless the
process is run with CAP_NET_ADMIN.

argp.prefer_busy_poll is a boolean field and must be either 0 (disabled) or 1 (enabled). If enabled,
this indicates to the network stack that busy poll is the preferred method of processing network data and
the network stack should give the application the opportunity to busy poll. Without this option, very
busy systems may continue to do network processing via the normal method of IRQs triggering softIRQ and
NAPI.

argp.__pad must be zero.

RETURN VALUE

       On success, 0 is returned.  On failure, -1 is returned, and errno is set to indicate the error.

ERRORS

       EOPNOTSUPP
              The kernel was not compiled with busy poll support.

       EINVAL fd is not a valid file descriptor.

       EINVAL argp.__pad is not zero.

       EINVAL argp.busy_poll_usecs exceeds INT_MAX.

       EINVAL argp.prefer_busy_poll is not 0 or 1.

       EPERM  The  process  is  being  run without CAP_NET_ADMIN and the specified argp.busy_poll_budget exceeds
              NAPI_POLL_WEIGHT.

       EFAULT argp is an invalid address.

STANDARDS

       Linux.

HISTORY

       Linux 6.9.  glibc 2.40.

EXAMPLES

       /* Code to set the epoll params to enable busy polling */

       int epollfd = epoll_create1(0);
       struct epoll_params params;

       if (epollfd == -1) {
           perror("epoll_create1");
           exit(EXIT_FAILURE);
       }

       memset(&params, 0, sizeof(struct epoll_params));

       params.busy_poll_usecs = 25;
       params.busy_poll_budget = 8;
       params.prefer_busy_poll = 1;

       if (ioctl(epollfd, EPIOCSPARAMS, &params) == -1) {
           perror("ioctl");
           exit(EXIT_FAILURE);
       }

       /* Code to show how to retrieve the current settings */

       memset(&params, 0, sizeof(struct epoll_params));

       if (ioctl(epollfd, EPIOCGPARAMS, &params) == -1) {
           perror("ioctl");
           exit(EXIT_FAILURE);
       }

       /* params struct now contains the current parameters */

       fprintf(stderr, "epoll usecs: %lu\n", params.busy_poll_usecs);
       fprintf(stderr, "epoll packet budget: %u\n", params.busy_poll_budget);
       fprintf(stderr, "epoll prefer busy poll: %u\n", params.prefer_busy_poll);