Linux AIO: Poor Scaling

Question

I am writing a library that uses the Linux asynchronous I/O system calls, and would like to know why the io_submit function is exhibiting poor scaling on the ext4 file system. If possible, what can I do to get io_submit not to block for large IO request sizes? I already do the following (as described here):

• Use O_DIRECT.
• Align the IO buffer to a 512-byte boundary.
• Set the buffer size to a multiple of the page size.

In order to observe how long the kernel spends in io_submit, I ran a test in which I created a 1 Gb test file using dd and /dev/urandom, and repeatedly dropped the system cache (sync; echo 1 > /proc/sys/vm/drop_caches) and read increasingly larger portions of the file. At each iteration, I printed the time taken by io_submit and the time spent waiting for the read request to finish. I ran the following experiment on an x86-64 system running Arch Linux, with kernel version 3.11. The machine has an SSD and a Core i7 CPU. The first graph plots the number of pages read against the time spent waiting for io_submit to finish. The second graph displays the time spent waiting for the read request to finish. The times are measured in seconds.

For comparison, I created a similar test that uses synchronous IO by means of pread. Here are the results:

It seems that the asynchronous IO works as expected up to request sizes of around 20,000 pages. After that, io_submit blocks. These observations lead to the following questions:

• Why isn't the execution time of io_submit constant?
• What is causing this poor scaling behavior?
• Do I need to split up all read requests on ext4 file systems into multiple requests, each of size less than 20,000 pages?
• Where does this "magic" value of 20,000 come from? If I run my program on another Linux system, how can I determine the largest IO request size to use without experiencing poor scaling behavior?

The code used to test the asynchronous IO follows below. I can add other source listings if you think they are relevant, but I tried to post only the details that I thought might be relevant.

#include <cstddef>
#include <cstdint>
#include <cstring>
#include <chrono>
#include <iostream>
#include <memory>
#include <fcntl.h>
#include <stdio.h>
#include <time.h>
#include <unistd.h>
// For `__NR_*` system call definitions.
#include <sys/syscall.h>
#include <linux/aio_abi.h>

static int
io_setup(unsigned n, aio_context_t* c)
{
    return syscall(__NR_io_setup, n, c);
}

static int
io_destroy(aio_context_t c)
{
    return syscall(__NR_io_destroy, c);
}

static int
io_submit(aio_context_t c, long n, iocb** b)
{
    return syscall(__NR_io_submit, c, n, b);
}

static int
io_getevents(aio_context_t c, long min, long max, io_event* e, timespec* t)
{
    return syscall(__NR_io_getevents, c, min, max, e, t);
}

int main(int argc, char** argv)
{
    using namespace std::chrono;
    const auto n = 4096 * size_t(std::atoi(argv[1]));

    // Initialize the file descriptor. If O_DIRECT is not used, the kernel
    // will block on `io_submit` until the job finishes, because non-direct
    // IO via the `aio` interface is not implemented (to my knowledge).
    auto fd = ::open("dat/test.dat", O_RDONLY | O_DIRECT | O_NOATIME);
    if (fd < 0) {
        ::perror("Error opening file");
        return EXIT_FAILURE;
    }

    char* p;
    auto r = ::posix_memalign((void**)&p, 512, n);
    if (r != 0) {
        std::cerr << "posix_memalign failed." << std::endl;
        return EXIT_FAILURE;
    }
    auto del = [](char* p) { std::free(p); };
    std::unique_ptr<char[], decltype(del)> buf{p, del};

    // Initialize the IO context.
    aio_context_t c{0};
    r = io_setup(4, &c);
    if (r < 0) {
        ::perror("Error invoking io_setup");
        return EXIT_FAILURE;
    }

    // Setup I/O control block.
    iocb b;
    std::memset(&b, 0, sizeof(b));
    b.aio_fildes = fd;
    b.aio_lio_opcode = IOCB_CMD_PREAD;

    // Command-specific options for `pread`.
    b.aio_buf = (uint64_t)buf.get();
    b.aio_offset = 0;
    b.aio_nbytes = n;
    iocb* bs[1] = {&b};

    auto t1 = high_resolution_clock::now();
    auto r = io_submit(c, 1, bs);
    if (r != 1) {
        if (r == -1) {
            ::perror("Error invoking io_submit");
        }
        else {
            std::cerr << "Could not submit request." << std::endl;
        }
        return EXIT_FAILURE;
    }
    auto t2 = high_resolution_clock::now();
    auto count = duration_cast<duration<double>>(t2 - t1).count();
    // Print the wait time.
    std::cout << count << " ";

    io_event e[1];
    t1 = high_resolution_clock::now();
    r = io_getevents(c, 1, 1, e, NULL);
    t2 = high_resolution_clock::now();
    count = duration_cast<duration<double>>(t2 - t1).count();
    // Print the read time.
    std::cout << count << std::endl;

    r = io_destroy(c);
    if (r < 0) {
        ::perror("Error invoking io_destroy");
        return EXIT_FAILURE;
    }
}

Answer 1

My understanding is that very few (if any) filesystems on linux fully supports AIO. Some filesystem operations still block, and sometimes io_submit() will, indirectly via filesystem operations, invoke such blocking calls.

My understanding is further that the main users of kernel AIO primarily care about AIO being truly asynchronous on raw block devices (i.e. no filesystem). essentially database vendors.

Here's a relevant post from the linux-aio mailing list. (head of the thread)

A possibly useful recommendation:

Add more requests via /sys/block/xxx/queue/nr_requests and the problem will get better.

Answer 2

Why isn't the execution time of io_submit constant?

Because you are submitting I/Os that are so big, the block layer has to split them up and then queue the resulting requests. This can then cause you to hit resource limitations that in turn cause io_submit() to behave as if it's blocking...

What is causing this poor scaling behavior?

The bigger the I/O is over the splitting threshold (see below) the more likely it becomes that the number of splits done to turn it into appropriately sized requests will also increase (presumably actually doing the splits will cost a small amount of time too). With direct I/O io_submit() does not return until all its requests have been allocated and queued at the block layer level. Further, the amount of requests that can be queued by the block layer for a given disk is limited to /sys/block/[disk_device]/queue/nr_requests. Exceeding this limit leads to io_submit() blocking until enough request slots have been freed up such that all its allocations have been satisfied (this is related to Arvid was recommending).

Do I need to split up all read requests on ext4 file systems into multiple requests, each of size less than 20,000 pages?

Ideally you should split your requests into far smaller amounts than that - 20000 pages (assuming a 4096 byte page which is what is used on x86 platforms) is roughly 78 megabytes! This doesn't just apply to when you're using ext4 - doing such large io_submit() I/O sizes to other filesystems or even directly to block devices will be unlikely perform well.

If you work out which disk device your filesystem is on and look at /sys/block/[disk_device]/queue/max_sectors_kb that will give you an upper bound but the bound at which splitting starts may be even smaller so you may want to limit the size of each I/O to /sys/block/[disk_device]/queue/max_segments * PAGE_SIZE instead.

Where does this "magic" value of 20,000 come from?

This is likely down to some combination of:

• The maximum size each I/O can be before the block layer splits it (at most this will be /sys/block/[disk_device]/queue/max_sectors_kb but the observed split limit may be even lower)
• The maximum number of I/Os that can be queued before blocking occurs (/sys/block/[disk_device]/queue/nr_requests)
• Your hardware's command queue depth (/sys/block/[disk_device]/device/queue_depth)
• How fast your disk is at completing requests. When the kernel can't queue any more I/Os to the real device (due to the hardware queue_depth being full and the kernel's additional queues being full) it becomes blocking on new requests until in-flight ones sent to the hardware have completed.

If I run my program on another Linux system, how can I determine the largest IO request size to use without experiencing poor scaling behavior?

Limit each request I/O to the lower of /sys/block/[disk_device]/queue/max_sectors_kb or /sys/block/[disk_device]/queue/max_segments * PAGE_SIZE. I would imagine I/Os no bigger than 524288 bytes should be safe but your hardware may be able to cope with a larger size and thus get a higher throughput but possibly at the expense of completion (as opposed to submission) latency.

If possible, what can I do to get io_submit not to block for large IO request sizes?

There's going to be an upper "good" limit and if you surpass it there are going to be consequences which you can't escape.

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