Is io_uring framework becoming bloated gradually? and introduces performace regression

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

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hi,

This issue was found when I tested IORING_FEAT_FAST_POLL feature, with the newest
upstream codes, indeed I find that io_uring's performace improvement is not obvious
compared to epoll in my test environment, most of the time they are similar.
Test cases basically comes from:
   https://github.com/frevib/io_uring-echo-server/blob/io-uring-feat-fast-poll/benchmarks/benchmarks.md.
In above url, the author's test results shows that io_uring will get a big performace
improvement compared to epoll. I'm still looking into why I don't get the big improvement,
currently don't know why, but I find some obvious regression issue.

I wrote a simple tool based io_uring nop operation to evaluate io_uring framework in
v5.1 and 5.7.0-rc4+(jens's io_uring-5.7 branch), I see a obvious performace regression:
v5.1 kernel:
     $sudo taskset -c 60 ./io_uring_nop_stress -r 300 # run 300 seconds
     total ios: 1832524960
     IOPS:      6108416
5.7.0-rc4+
     $sudo taskset -c 60 ./io_uring_nop_stress -r 300
     total ios: 1597672304
     IOPS:      5325574
it's about 12% performance regression.

Using perf can see many performance bottlenecks, for example, io_submit_sqes is one.
For now, I did't make many analysis yet, just have a look at io_submit_sqes(), there
are many assignment operations in io_init_req(), but I'm not sure whether they are all
needed when req is not needed to be punt to io-wq, for example,
     INIT_IO_WORK(&req->work, io_wq_submit_work); # a whole struct assignment
from perf annotate tool, it's an expensive operation, I think reqs that use fast poll
feature use task-work function, so the INIT_IO_WORK maybe not necessary.

Above is just one issue, what I worry is that whether io_uring is becoming more bloated
gradually, and will not that better to aio. In https://kernel.dk/io_uring.pdf, it says
that io_uring will eliminate 104 bytes copy compared to aio, but see currenct
io_init_req(), io_uring maybe copy more, introducing more overhead? Or does we need to
carefully re-design struct io_kiocb, to reduce overhead as soon as possible.


Regards,
Xiaoguang Wang

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/*
 * Description: run various io_uring nop tests
 * xiaoguang.wang@linux.alibaba.com
 *
 */
#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>

#include "liburing.h"

static char *myprog;
static int batch_count = 16;
static int force_async = 0;
static int runtime = 30;
static unsigned long long ios;
static volatile int stop = 0;

static int test_nop(struct io_uring *ring)
{
	struct io_uring_cqe *cqe;
	struct io_uring_sqe *sqe;
	int i, ret;

	for (i = 0; i < batch_count; i++) {
		sqe = io_uring_get_sqe(ring);
		if (!sqe) {
			fprintf(stderr, "get sqe failed\n");
			goto err;
		}
		io_uring_prep_nop(sqe);
		if (force_async)
			sqe->flags |= IOSQE_ASYNC;
	}

	ret = io_uring_submit(ring);
	if (ret <= 0) {
		fprintf(stderr, "sqe submit failed: %d\n", ret);
		goto err;
	}

	for (i = 0; i < batch_count; i++) {
		ret = io_uring_wait_cqe(ring, &cqe);
		if (ret < 0) {
			fprintf(stderr, "wait completion %d\n", ret);
			goto err;
		}
		io_uring_cqe_seen(ring, cqe);
	}
	ios += batch_count;
	return 0;
err:
	return 1;
}

static void usage(void)
{
	printf("Usage: %s -H   or\n", myprog);
	printf("       %s [-b batch][-a][-r runtime]\n", myprog);
	printf("   -b  batch    submission batch count, default 16\n");
	printf("   -r  runtime  run time, default 30\n");
	printf("   -a           force asynchronous submission\n");
	printf("   -H           prints usage and exits\n");
}

static void alarm_handler(int signum)
{
	(void)signum;
	stop = 1;
}

int main(int argc, char *argv[])
{
	struct io_uring ring;
	struct sigaction sa;
	int ret, c;
	const char *opts = "b:ar:";

	myprog = argv[0];
	while ((c = getopt(argc, argv, opts)) != -1) {
		switch (c) {
		case 'b':
			batch_count = atoi(optarg);
			break;
                case 'a':
			force_async = 1;
			break;
                case 'r':
			runtime = atoi(optarg);
			break;
		case 'H':
			usage();
			exit(1);
		}
	}

	if (!batch_count) {
		fprintf(stderr, "batch count should be greater than 0\n");
		exit(1);
	}

	if (!runtime) {
		printf("run time is zero, are you sure?\n");
		return 0;
	}

	memset(&sa, 0, sizeof(sa));
	sa.sa_handler = alarm_handler;
	sigemptyset(&sa.sa_mask);
	ret = sigaction(SIGALRM, &sa, NULL);
	if (ret < 0) {
		fprintf(stderr, "sigaction failed: %s", strerror(errno));
		exit(1);
	}
	alarm(runtime);

	ret = io_uring_queue_init(batch_count, &ring, 0);
	if (ret) {
		fprintf(stderr, "ring setup failed: %d\n", ret);
		return 1;
	}

	while (!stop) {
		ret = test_nop(&ring);
		if (ret) {
			fprintf(stderr, "test_nop failed\n");
			return ret;
		}
	}
	printf("total ios: %llu\n", ios);
	printf("IOPS:      %llu\n", ios / runtime);

	return 0;
}