Re: [ISSUE] The time cost of IOSQE_IO_LINK

Carter Li 李通洲
2020-02-13T00:33:42Z

Thanks for your reply.

You are right the nop isn't really a good test case. But I actually
found this issue when benchmarking my echo server, which didn't use
NOP of course.

Test case attached below. Use rust_echo_bench for benchmarking.
https://github.com/haraldh/rust_echo_bench

$ gcc link_recv.c -o link_recv -luring -O3 -DUSE_LINK=0
$ ./link_recv 12345
$ cargo run --release # On another console
Benchmarking: 127.0.0.1:12345
50 clients, running 512 bytes, 60 sec.

Speed: 168264 request/sec, 168264 response/sec
Requests: 10095846
Responses: 10095844

$ gcc link_recv.c -o link_recv -luring -O3 -DUSE_LINK=1
$ ./link_recv 12345
$ cargo run --release # On another console
Benchmarking: 127.0.0.1:12345
50 clients, running 512 bytes, 60 sec.

Speed: 112666 request/sec, 112666 response/sec
Requests: 6760009
Responses: 6759975

I think `POLL_ADD(POLLIN)-RECV` and `POLL_ADD(POLLOUT)-SEND` are common use cases for networking ( for some reason a short read for SEND is not considered an error, `RECV-SEND` cannot be used in a link chain ). RECV/SEND won't block after polled. I expect better performance for fewer io_uring_enter syscalls. Could you please have a check with it?

Another more complex test case `POLL_ADD-READ_FIXED-WRITE_FIXED` I have posted on Github, which currently results in freeze.

https://github.com/axboe/liburing/issues/71

Carter

---

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <sys/socket.h>
#include <sys/poll.h>
#include <netinet/in.h>

#include <liburing.h>

#define BACKLOG 128
#define MAX_MESSAGE_LEN 1024
#define MAX_CONNECTIONS 1024
#ifndef USE_LINK
# define USE_LINK 0
#endif

enum { ACCEPT, POLL, READ, WRITE };

struct conn_info {
__u32 fd;
__u32 type;
};

typedef char buf_type[MAX_CONNECTIONS][MAX_MESSAGE_LEN];

static struct io_uring ring;
static unsigned cqe_count = 0;

int init_socket(int portno) {
int sock_listen_fd = socket(AF_INET, SOCK_STREAM, 0);
if (sock_listen_fd < 0) {
perror("socket");
return -1;
}

struct sockaddr_in server_addr = {
.sin_family = AF_INET,
.sin_port = htons(portno),
.sin_addr = {
.s_addr = INADDR_ANY,
},
};

if (bind(sock_listen_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
perror("bind");
return -1;
}

if (listen(sock_listen_fd, BACKLOG) < 0) {
perror("listen");
return -1;
}

return sock_listen_fd;
}

static struct io_uring_sqe* get_sqe_safe() {
struct io_uring_sqe *sqe = io_uring_get_sqe(&ring);
if (__builtin_expect(!!sqe, 1)) {
return sqe;
} else {
io_uring_cq_advance(&ring, cqe_count);
cqe_count = 0;
io_uring_submit(&ring);
return io_uring_get_sqe(&ring);
}
}

static void add_accept(int fd, struct sockaddr *client_addr, socklen_t *client_len) {
struct io_uring_sqe *sqe = get_sqe_safe();
struct conn_info conn_i = {
.fd = fd,
.type = ACCEPT,
};

io_uring_prep_accept(sqe, fd, client_addr, client_len, 0);
memcpy(&sqe->user_data, &conn_i, sizeof(conn_i));
}

static void add_poll(int fd, int poll_mask, unsigned flags) {
struct io_uring_sqe *sqe = get_sqe_safe();
struct conn_info conn_i = {
.fd = fd,
.type = POLL,
};

io_uring_prep_poll_add(sqe, fd, poll_mask);
io_uring_sqe_set_flags(sqe, flags);
memcpy(&sqe->user_data, &conn_i, sizeof(conn_i));
}

static void add_socket_read(int fd, size_t size, buf_type *bufs) {
struct io_uring_sqe *sqe = get_sqe_safe();
struct conn_info conn_i = {
.fd = fd,
.type = READ,
};

io_uring_prep_recv(sqe, fd, (*bufs)[fd], size, MSG_NOSIGNAL);
memcpy(&sqe->user_data, &conn_i, sizeof(conn_i));
}

static void add_socket_write(int fd, size_t size, buf_type *bufs, unsigned flags) {
struct io_uring_sqe *sqe = get_sqe_safe();
struct conn_info conn_i = {
.fd = fd,
.type = WRITE,
};

io_uring_prep_send(sqe, fd, (*bufs)[fd], size, MSG_NOSIGNAL);
io_uring_sqe_set_flags(sqe, flags);
memcpy(&sqe->user_data, &conn_i, sizeof(conn_i));
}

int main(int argc, char *argv[]) {
if (argc < 2) {
fprintf(stderr, "Please give a port number: %s [port]\n", argv[0]);
return 1;
}

int portno = strtol(argv[1], NULL, 10);
int sock_listen_fd = init_socket(portno);
if (sock_listen_fd < 0) return -1;
printf("io_uring echo server listening for connections on port: %d\n", portno);

int ret = io_uring_queue_init(BACKLOG, &ring, 0);
if (ret < 0) {
fprintf(stderr, "queue_init: %s\n", strerror(-ret));
return -1;
}

buf_type *bufs = (buf_type *)malloc(sizeof(*bufs));

struct sockaddr_in client_addr;
socklen_t client_len = sizeof(client_addr);
add_accept(sock_listen_fd, (struct sockaddr *)&client_addr, &client_len);

while (1) {
io_uring_submit_and_wait(&ring, 1);

struct io_uring_cqe *cqe;
unsigned head;

io_uring_for_each_cqe(&ring, head, cqe) {
++cqe_count;

struct conn_info conn_i;
memcpy(&conn_i, &cqe->user_data, sizeof(conn_i));
int result = cqe->res;

switch (conn_i.type) {
case ACCEPT:
#if USE_LINK
add_poll(result, POLLIN, IOSQE_IO_LINK);
add_socket_read(result, MAX_MESSAGE_LEN, bufs);
#else
add_poll(result, POLLIN, 0);
#endif
add_accept(sock_listen_fd, (struct sockaddr *)&client_addr, &client_len);
break;

#if !USE_LINK
case POLL:
add_socket_read(conn_i.fd, MAX_MESSAGE_LEN, bufs);
break;
#endif

case READ:
if (__builtin_expect(result <= 0, 0)) {
shutdown(conn_i.fd, SHUT_RDWR);
} else {
add_socket_write(conn_i.fd, result, bufs, 0);
}
break;

case WRITE:
#if USE_LINK
add_poll(conn_i.fd, POLLIN, IOSQE_IO_LINK);
add_socket_read(conn_i.fd, MAX_MESSAGE_LEN, bufs);
#else
add_poll(conn_i.fd, POLLIN, 0);
#endif
break;
}
}

io_uring_cq_advance(&ring, cqe_count);
cqe_count = 0;
}

close(sock_listen_fd);
free(bufs);
}

> 2020年2月13日 上午1:11，Jens Axboe <axboe@kernel.dk> 写道：
>
> On 2/12/20 9:31 AM, Carter Li 李通洲 wrote:
>> Hi everyone,
>>
>> IOSQE_IO_LINK seems to have very high cost, even greater then io_uring_enter syscall.
>>
>> Test code attached below. The program completes after getting 100000000 cqes.
>>
>> $ gcc test.c -luring -o test0 -g -O3 -DUSE_LINK=0
>> $ time ./test0
>> USE_LINK: 0, count: 100000000, submit_count: 1562500
>> 0.99user 9.99system 0:11.02elapsed 99%CPU (0avgtext+0avgdata 1608maxresident)k
>> 0inputs+0outputs (0major+72minor)pagefaults 0swaps
>>
>> $ gcc test.c -luring -o test1 -g -O3 -DUSE_LINK=1
>> $ time ./test1
>> USE_LINK: 1, count: 100000110, submit_count: 799584
>> 0.83user 19.21system 0:20.90elapsed 95%CPU (0avgtext+0avgdata 1632maxresident)k
>> 0inputs+0outputs (0major+72minor)pagefaults 0swaps
>>
>> As you can see, the `-DUSE_LINK=1` version emits only about half io_uring_submit calls
>> of the other version, but takes twice as long. That makes IOSQE_IO_LINK almost useless,
>> please have a check.
>
> The nop isn't really a good test case, as it doesn't contain any smarts
> in terms of executing a link fast. So it doesn't say a whole lot outside
> of "we could make nop links faster", which is also kind of pointless.
>
> "Normal" commands will work better. Where the link is really a win is if
> the first request needs to go async to complete. For that case, the
> next link can execute directly from that context. This saves an async
> punt for the common case.
>
> --
> Jens Axboe
>