Re: [PATCH 0/2] Add epoll round robin wakeup mode

[CC += linux-api@vger.kernel.org]

Jason,

Since this is a kernel-user-space API change, please CC linux-api@.
The kernel source file Documentation/SubmitChecklist notes that all
Linux kernel patches that change userspace interfaces should be CCed
to linux-api@vger.kernel.org, so that the various parties who are
interested in API changes are informed. For further information, see
https://www.kernel.org/doc/man-pages/linux-api-ml.html


Thanks,

Michael


On Mon, Feb 9, 2015 at 9:05 PM, Jason Baron [off-list ref] wrote:

Hi,

When we are sharing a wakeup source among multiple epoll fds, we end up with
thundering herd wakeups, since there is currently no way to add to the
wakeup source exclusively. This series introduces 2 new epoll flags,
EPOLLEXCLUSIVE for adding to a wakeup source exclusively. And EPOLLROUNDROBIN
which is to be used in conjunction to EPOLLEXCLUSIVE to evenly
distribute the wakeups. I'm showing perf results from the simple pipe() usecase
below. But this patch was originally motivated by a desire to improve
wakeup balance and cpu usage for a shared listen socket().

Perf stat, 3.19.0-rc7+, 4 core, Intel(R) Xeon(R) CPU E3-1265L v3 @ 2.50GHz:

pipe test wake all:

 Performance counter stats for './wake':

      10837.480396      task-clock (msec)         #    1.879 CPUs utilized
           2047108      context-switches          #    0.189 M/sec
            214491      cpu-migrations            #    0.020 M/sec
               247      page-faults               #    0.023 K/sec
       23655687888      cycles                    #    2.183 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
       11242141621      instructions              #    0.48  insns per cycle
        2313479486      branches                  #  213.470 M/sec
          13679036      branch-misses             #    0.59% of all branches

       5.768295821 seconds time elapsed

pipe test wake balanced:

 Performance counter stats for './wake -o':

        291.250312      task-clock (msec)         #    0.094 CPUs utilized
             40308      context-switches          #    0.138 M/sec
              1448      cpu-migrations            #    0.005 M/sec
               248      page-faults               #    0.852 K/sec
         646407197      cycles                    #    2.219 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
         364256883      instructions              #    0.56  insns per cycle
          65775397      branches                  #  225.838 M/sec
            535637      branch-misses             #    0.81% of all branches

       3.086694452 seconds time elapsed

Rough epoll manpage text:

EPOLLEXCLUSIVE
        Provides exclusive wakeups when attaching multiple epoll fds to a
        shared wakeup source. Must be specified on an EPOLL_CTL_ADD operation.

EPOLLROUNDROBIN
        Provides balancing for exclusive wakeups when attaching multiple epoll
        fds to a shared wakeup soruce. Must be specificed with EPOLLEXCLUSIVE
        during an EPOLL_CTL_ADD operation.


Thanks,

-Jason

#include <unistd.h>
#include <sys/epoll.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define NUM_THREADS 100
#define NUM_EVENTS 20000
#define EPOLLEXCLUSIVE (1 << 28)
#define EPOLLBALANCED (1 << 27)

int optimize, exclusive;
int p[2];
pthread_t threads[NUM_THREADS];
int event_count[NUM_THREADS];

struct epoll_event evt = {
        .events = EPOLLIN
};

void die(const char *msg) {
    perror(msg);
    exit(-1);
}

void *run_func(void *ptr)
{
        int i = 0;
        int j = 0;
        int ret;
        int epfd;
        char buf[4];
        int id = *(int *)ptr;
        int *contents;

        if ((epfd = epoll_create(1)) < 0)
                die("create");

        if (optimize)
                evt.events |= ((EPOLLBALANCED | EPOLLEXCLUSIVE));
        else if (exclusive)
                evt.events |= EPOLLEXCLUSIVE;
        ret = epoll_ctl(epfd, EPOLL_CTL_ADD, p[0], &evt);
        if (ret)
                perror("epoll_ctl add error!\n");

        while (1) {
                ret = epoll_wait(epfd, &evt, 10000, -1);
                ret = read(p[0], buf, sizeof(int));
                if (ret == 4)
                        event_count[id]++;
        }
}

int main(int argc, char *argv[])
{
        int ret, i, j;
        int id[NUM_THREADS];
        int total = 0;
        int nohit = 0;
        int extra_wakeups = 0;

        if (argc == 2) {
                if (strcmp(argv[1], "-o") == 0)
                        optimize = 1;
                if (strcmp(argv[1], "-e") == 0)
                        exclusive = 1;
        }

        if (pipe(p) < 0)
                die("pipe");

        for (i = 0; i < NUM_THREADS; i++) {
                id[i] = i;
                pthread_create(&threads[i], NULL, run_func, &id[i]);
        }

        for (j = 0; j < NUM_EVENTS; j++) {
                write(p[1], p, sizeof(int));
                usleep(100);
        }

        for (i = 0; i < NUM_THREADS; i++) {
                pthread_cancel(threads[i]);
                printf("joined: %d\n", i);
                printf("event count: %d\n", event_count[i]);
                total += event_count[i];
                if (!event_count[i])
                        nohit++;
        }

        printf("total events is: %d\n", total);
        printf("nohit is: %d\n", nohit);
}


Jason Baron (2):
  sched/wait: add round robin wakeup mode
  epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

 fs/eventpoll.c                 | 25 ++++++++++++++++++++-----
 include/linux/wait.h           | 11 +++++++++++
 include/uapi/linux/eventpoll.h |  6 ++++++
 kernel/sched/wait.c            |  5 ++++-
 4 files changed, 41 insertions(+), 6 deletions(-)

--
1.8.2.rc2

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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/