Re: [Qemu-devel] [PATCH 10/12] migration: introduce lockless multithreads model

[Peter Xu]

On Mon, Jun 04, 2018 at 05:55:18PM +0800, address@hidden wrote:
> From: Xiao Guangrong <address@hidden>
> 
> Current implementation of compression and decompression are very
> hard to be enabled on productions. We noticed that too many wait-wakes
> go to kernel space and CPU usages are very low even if the system
> is really free
> 
> The reasons are:
> 1) there are two many locks used to do synchronous，there
> 　　is a global lock and each single thread has its own lock,
> 　　migration thread and work threads need to go to sleep if
> 　　these locks are busy
> 
> 2) migration thread separately submits request to the thread
>    however, only one request can be pended, that means, the
>    thread has to go to sleep after finishing the request
> 
> To make it work better, we introduce a new multithread model,
> the user, currently it is the migration thread, submits request
> to each thread with round-robin manner, the thread has its own
> ring whose capacity is 4 and puts the result to a global ring
> which is lockless for multiple producers, the user fetches result
> out from the global ring and do remaining operations for the
> request, e.g, posting the compressed data out for migration on
> the source QEMU
> 
> Performance Result:
> The test was based on top of the patch:
>    ring: introduce lockless ring buffer
> that means, previous optimizations are used for both of original case
> and applying the new multithread model
> 
> We tested live migration on two hosts:
>    Intel(R) Xeon(R) Gold 6142 CPU @ 2.60GHz * 64 + 256G memory
> to migration a VM between each other, which has 16 vCPUs and 60G
> memory, during the migration, multiple threads are repeatedly writing
> the memory in the VM
> 
> We used 16 threads on the destination to decompress the data and on the
> source, we tried 8 threads and 16 threads to compress the data
> 
> --- Before our work ---
> migration can not be finished for both 8 threads and 16 threads. The data
> is as followings:
> 
> Use 8 threads to compress:
> - on the source:
>           migration thread   compress-threads
> CPU usage       70%          some use 36%, others are very low ~20%
> - on the destination:
>             main thread        decompress-threads
> CPU usage       100%         some use ~40%, other are very low ~2%
> 
> Migration status (CAN NOT FINISH):
> info migrate
> globals:
> store-global-state: on
> only-migratable: off
> send-configuration: on
> send-section-footer: on
> capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: 
> off compress: on events: off postcopy-ram: off x-colo: off release-ram: off 
> block: off return-path: off pause-before-switchover: off x-multifd: off 
> dirty-bitmaps: off postcopy-blocktime: off
> Migration status: active
> total time: 1019540 milliseconds
> expected downtime: 2263 milliseconds
> setup: 218 milliseconds
> transferred ram: 252419995 kbytes
> throughput: 2469.45 mbps
> remaining ram: 15611332 kbytes
> total ram: 62931784 kbytes
> duplicate: 915323 pages
> skipped: 0 pages
> normal: 59673047 pages
> normal bytes: 238692188 kbytes
> dirty sync count: 28
> page size: 4 kbytes
> dirty pages rate: 170551 pages
> compression pages: 121309323 pages
> compression busy: 60588337
> compression busy rate: 0.36
> compression reduced size: 484281967178
> compression rate: 0.97
> 
> Use 16 threads to compress:
> - on the source:
>           migration thread   compress-threads
> CPU usage       96%          some use 45%, others are very low ~6%
> - on the destination:
>             main thread        decompress-threads
> CPU usage       96%         some use 58%, other are very low ~10%
> 
> Migration status (CAN NOT FINISH):
> info migrate
> globals:
> store-global-state: on
> only-migratable: off
> send-configuration: on
> send-section-footer: on
> capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: 
> off compress: on events: off postcopy-ram: off x-colo: off release-ram: off 
> block: off return-path: off pause-before-switchover: off x-multifd: off 
> dirty-bitmaps: off postcopy-blocktime: off
> Migration status: active
> total time: 1189221 milliseconds
> expected downtime: 6824 milliseconds
> setup: 220 milliseconds
> transferred ram: 90620052 kbytes
> throughput: 840.41 mbps
> remaining ram: 3678760 kbytes
> total ram: 62931784 kbytes
> duplicate: 195893 pages
> skipped: 0 pages
> normal: 17290715 pages
> normal bytes: 69162860 kbytes
> dirty sync count: 33
> page size: 4 kbytes
> dirty pages rate: 175039 pages
> compression pages: 186739419 pages
> compression busy: 17486568
> compression busy rate: 0.09
> compression reduced size: 744546683892
> compression rate: 0.97
> 
> --- After our work ---
> Migration can be finished quickly for both 8 threads and 16 threads. The
> data is as followings:
> 
> Use 8 threads to compress:
> - on the source:
>           migration thread   compress-threads
> CPU usage       30%               30% (all threads have same CPU usage)
> - on the destination:
>             main thread        decompress-threads
> CPU usage       100%              50% (all threads have same CPU usage)
> 
> Migration status (finished in 219467 ms):
> info migrate
> globals:
> store-global-state: on
> only-migratable: off
> send-configuration: on
> send-section-footer: on
> capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: 
> off compress: on events: off postcopy-ram: off x-colo: off release-ram: off 
> block: off return-path: off pause-before-switchover: off x-multifd: off 
> dirty-bitmaps: off postcopy-blocktime: off
> Migration status: completed
> total time: 219467 milliseconds
> downtime: 115 milliseconds
> setup: 222 milliseconds
> transferred ram: 88510173 kbytes
> throughput: 3303.81 mbps
> remaining ram: 0 kbytes
> total ram: 62931784 kbytes
> duplicate: 2211775 pages
> skipped: 0 pages
> normal: 21166222 pages
> normal bytes: 84664888 kbytes
> dirty sync count: 15
> page size: 4 kbytes
> compression pages: 32045857 pages
> compression busy: 23377968
> compression busy rate: 0.34
> compression reduced size: 127767894329
> compression rate: 0.97
> 
> Use 16 threads to compress:
> - on the source:
>           migration thread   compress-threads
> CPU usage       60%               60% (all threads have same CPU usage)
> - on the destination:
>             main thread        decompress-threads
> CPU usage       100%              75% (all threads have same CPU usage)
> 
> Migration status (finished in 64118 ms):
> info migrate
> globals:
> store-global-state: on
> only-migratable: off
> send-configuration: on
> send-section-footer: on
> capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: 
> off compress: on events: off postcopy-ram: off x-colo: off release-ram: off 
> block: off return-path: off pause-before-switchover: off x-multifd: off 
> dirty-bitmaps: off postcopy-blocktime: off
> Migration status: completed
> total time: 64118 milliseconds
> downtime: 29 milliseconds
> setup: 223 milliseconds
> transferred ram: 13345135 kbytes
> throughput: 1705.10 mbps
> remaining ram: 0 kbytes
> total ram: 62931784 kbytes
> duplicate: 574921 pages
> skipped: 0 pages
> normal: 2570281 pages
> normal bytes: 10281124 kbytes
> dirty sync count: 9
> page size: 4 kbytes
> compression pages: 28007024 pages
> compression busy: 3145182
> compression busy rate: 0.08
> compression reduced size: 111829024985
> compression rate: 0.97

Not sure how other people think, for me these information suites
better as cover letter.  For commit message, I would prefer to know
about something like: what this thread model can do; how the APIs are
designed and used; what's the limitations, etc.  After all until this
patch nowhere is using the new model yet, so these numbers are a bit
misleading.

> 
> Signed-off-by: Xiao Guangrong <address@hidden>
> ---
>  migration/Makefile.objs |   1 +
>  migration/threads.c     | 265 
> ++++++++++++++++++++++++++++++++++++++++++++++++
>  migration/threads.h     | 116 +++++++++++++++++++++

Again, this model seems to be suitable for scenarios even outside
migration.  So I'm not sure whether you'd like to generalize it (I
still see e.g. constants and comments related to migration, but there
aren't much) and put it into util/.

>  3 files changed, 382 insertions(+)
>  create mode 100644 migration/threads.c
>  create mode 100644 migration/threads.h
> 
> diff --git a/migration/Makefile.objs b/migration/Makefile.objs
> index c83ec47ba8..bdb61a7983 100644
> --- a/migration/Makefile.objs
> +++ b/migration/Makefile.objs
> @@ -7,6 +7,7 @@ common-obj-y += qemu-file-channel.o
>  common-obj-y += xbzrle.o postcopy-ram.o
>  common-obj-y += qjson.o
>  common-obj-y += block-dirty-bitmap.o
> +common-obj-y += threads.o
>  
>  common-obj-$(CONFIG_RDMA) += rdma.o
>  
> diff --git a/migration/threads.c b/migration/threads.c
> new file mode 100644
> index 0000000000..eecd3229b7
> --- /dev/null
> +++ b/migration/threads.c
> @@ -0,0 +1,265 @@
> +#include "threads.h"
> +
> +/* retry to see if there is avilable request before actually go to wait. */
> +#define BUSY_WAIT_COUNT 1000
> +
> +static void *thread_run(void *opaque)
> +{
> +    ThreadLocal *self_data = (ThreadLocal *)opaque;
> +    Threads *threads = self_data->threads;
> +    void (*handler)(ThreadRequest *data) = threads->thread_request_handler;
> +    ThreadRequest *request;
> +    int count, ret;
> +
> +    for ( ; !atomic_read(&self_data->quit); ) {
> +        qemu_event_reset(&self_data->ev);
> +
> +        count = 0;
> +        while ((request = ring_get(self_data->request_ring)) ||
> +            count < BUSY_WAIT_COUNT) {
> +             /*
> +             * wait some while before go to sleep so that the user
> +             * needn't go to kernel space to wake up the consumer
> +             * threads.
> +             *
> +             * That will waste some CPU resource indeed however it
> +             * can significantly improve the case that the request
> +             * will be available soon.
> +             */
> +             if (!request) {
> +                cpu_relax();
> +                count++;
> +                continue;
> +            }
> +            count = 0;
> +
> +            handler(request);
> +
> +            do {
> +                ret = ring_put(threads->request_done_ring, request);
> +                /*
> +                 * request_done_ring has enough room to contain all
> +                 * requests, however, theoretically, it still can be
> +                 * fail if the ring's indexes are overflow that would
> +                 * happen if there is more than 2^32 requests are

Could you elaborate why this ring_put() could fail, and why failure is
somehow related to 2^32 overflow?

Firstly, I don't understand why it will fail.

Meanwhile, AFAIU your ring can even live well with that 2^32 overflow.
Or did I misunderstood?

> +                 * handled between two calls of threads_wait_done().
> +                 * So we do retry to make the code more robust.
> +                 *
> +                 * It is unlikely the case for migration as the block's
> +                 * memory is unlikely more than 16T (2^32 pages) memory.

(some migration-related comments; maybe we can remove that)

> +                 */
> +                if (ret) {
> +                    fprintf(stderr,
> +                            "Potential BUG if it is triggered by 
> migration.\n");
> +                }
> +            } while (ret);
> +        }
> +
> +        qemu_event_wait(&self_data->ev);
> +    }
> +
> +    return NULL;
> +}
> +
> +static void add_free_request(Threads *threads, ThreadRequest *request)
> +{
> +    QSLIST_INSERT_HEAD(&threads->free_requests, request, node);
> +    threads->free_requests_nr++;
> +}
> +
> +static ThreadRequest *get_and_remove_first_free_request(Threads *threads)
> +{
> +    ThreadRequest *request;
> +
> +    if (QSLIST_EMPTY(&threads->free_requests)) {
> +        return NULL;
> +    }
> +
> +    request = QSLIST_FIRST(&threads->free_requests);
> +    QSLIST_REMOVE_HEAD(&threads->free_requests, node);
> +    threads->free_requests_nr--;
> +    return request;
> +}
> +
> +static void uninit_requests(Threads *threads, int free_nr)
> +{
> +    ThreadRequest *request;
> +
> +    /*
> +     * all requests should be released to the list if threads are being
> +     * destroyed, i,e. should call threads_wait_done() first.
> +     */
> +    assert(threads->free_requests_nr == free_nr);
> +
> +    while ((request = get_and_remove_first_free_request(threads))) {
> +        threads->thread_request_uninit(request);
> +    }
> +
> +    assert(ring_is_empty(threads->request_done_ring));
> +    ring_free(threads->request_done_ring);
> +}
> +
> +static int init_requests(Threads *threads)
> +{
> +    ThreadRequest *request;
> +    unsigned int done_ring_size = pow2roundup32(threads->total_requests);
> +    int i, free_nr = 0;
> +
> +    threads->request_done_ring = ring_alloc(done_ring_size,
> +                                            RING_MULTI_PRODUCER);
> +
> +    QSLIST_INIT(&threads->free_requests);
> +    for (i = 0; i < threads->total_requests; i++) {
> +        request = threads->thread_request_init();
> +        if (!request) {
> +            goto cleanup;
> +        }
> +
> +        free_nr++;
> +        add_free_request(threads, request);
> +    }
> +    return 0;
> +
> +cleanup:
> +    uninit_requests(threads, free_nr);
> +    return -1;
> +}
> +
> +static void uninit_thread_data(Threads *threads)
> +{
> +    ThreadLocal *thread_local = threads->per_thread_data;
> +    int i;
> +
> +    for (i = 0; i < threads->threads_nr; i++) {
> +        thread_local[i].quit = true;
> +        qemu_event_set(&thread_local[i].ev);
> +        qemu_thread_join(&thread_local[i].thread);
> +        qemu_event_destroy(&thread_local[i].ev);
> +        assert(ring_is_empty(thread_local[i].request_ring));
> +        ring_free(thread_local[i].request_ring);
> +    }
> +}
> +
> +static void init_thread_data(Threads *threads)
> +{
> +    ThreadLocal *thread_local = threads->per_thread_data;
> +    char *name;
> +    int i;
> +
> +    for (i = 0; i < threads->threads_nr; i++) {
> +        qemu_event_init(&thread_local[i].ev, false);
> +
> +        thread_local[i].threads = threads;
> +        thread_local[i].self = i;
> +        thread_local[i].request_ring = ring_alloc(threads->thread_ring_size, 
> 0);
> +        name = g_strdup_printf("%s/%d", threads->name, thread_local[i].self);
> +        qemu_thread_create(&thread_local[i].thread, name,
> +                           thread_run, &thread_local[i], 
> QEMU_THREAD_JOINABLE);
> +        g_free(name);
> +    }
> +}
> +
> +/* the size of thread local request ring */
> +#define THREAD_REQ_RING_SIZE 4
> +
> +Threads *threads_create(unsigned int threads_nr, const char *name,
> +                        ThreadRequest *(*thread_request_init)(void),
> +                        void (*thread_request_uninit)(ThreadRequest 
> *request),
> +                        void (*thread_request_handler)(ThreadRequest 
> *request),
> +                        void (*thread_request_done)(ThreadRequest *request))
> +{
> +    Threads *threads;
> +    int ret;
> +
> +    threads = g_malloc0(sizeof(*threads) + threads_nr * sizeof(ThreadLocal));
> +    threads->threads_nr = threads_nr;
> +    threads->thread_ring_size = THREAD_REQ_RING_SIZE;

(If we're going to generalize this thread model, maybe you'd consider
 to allow specify this ring size as well?)

> +    threads->total_requests = threads->thread_ring_size * threads_nr;
> +
> +    threads->name = name;
> +    threads->thread_request_init = thread_request_init;
> +    threads->thread_request_uninit = thread_request_uninit;
> +    threads->thread_request_handler = thread_request_handler;
> +    threads->thread_request_done = thread_request_done;
> +
> +    ret = init_requests(threads);
> +    if (ret) {
> +        g_free(threads);
> +        return NULL;
> +    }
> +
> +    init_thread_data(threads);
> +    return threads;
> +}
> +
> +void threads_destroy(Threads *threads)
> +{
> +    uninit_thread_data(threads);
> +    uninit_requests(threads, threads->total_requests);
> +    g_free(threads);
> +}
> +
> +ThreadRequest *threads_submit_request_prepare(Threads *threads)
> +{
> +    ThreadRequest *request;
> +    unsigned int index;
> +
> +    index = threads->current_thread_index % threads->threads_nr;

Why round-robin rather than simply find a idle thread (still with
valid free requests) and put the request onto that?

Asked since I don't see much difficulty to achieve that, meanwhile for
round-robin I'm not sure whether it can happen that one thread stuck
due to some reason (e.g., scheduling reason?), while the rest of the
threads are idle, then would threads_submit_request_prepare() be stuck
for that hanging thread?

> +
> +    /* the thread is busy */
> +    if (ring_is_full(threads->per_thread_data[index].request_ring)) {
> +        return NULL;
> +    }
> +
> +    /* try to get the request from the list */
> +    request = get_and_remove_first_free_request(threads);
> +    if (request) {
> +        goto got_request;
> +    }
> +
> +    /* get the request already been handled by the threads */
> +    request = ring_get(threads->request_done_ring);
> +    if (request) {
> +        threads->thread_request_done(request);
> +        goto got_request;
> +    }
> +    return NULL;
> +
> +got_request:
> +    threads->current_thread_index++;
> +    request->thread_index = index;
> +    return request;
> +}
> +
> +void threads_submit_request_commit(Threads *threads, ThreadRequest *request)
> +{
> +    int ret, index = request->thread_index;
> +    ThreadLocal *thread_local = &threads->per_thread_data[index];
> +
> +    ret = ring_put(thread_local->request_ring, request);
> +
> +    /*
> +     * we have detected that the thread's ring is not full in
> +     * threads_submit_request_prepare(), there should be free
> +     * room in the ring
> +     */
> +    assert(!ret);
> +    /* new request arrived, notify the thread */
> +    qemu_event_set(&thread_local->ev);
> +}
> +
> +void threads_wait_done(Threads *threads)
> +{
> +    ThreadRequest *request;
> +
> +retry:
> +    while ((request = ring_get(threads->request_done_ring))) {
> +        threads->thread_request_done(request);
> +        add_free_request(threads, request);
> +    }
> +
> +    if (threads->free_requests_nr != threads->total_requests) {
> +        cpu_relax();
> +        goto retry;
> +    }
> +}
> diff --git a/migration/threads.h b/migration/threads.h
> new file mode 100644
> index 0000000000..eced913065
> --- /dev/null
> +++ b/migration/threads.h
> @@ -0,0 +1,116 @@
> +#ifndef QEMU_MIGRATION_THREAD_H
> +#define QEMU_MIGRATION_THREAD_H
> +
> +/*
> + * Multithreads abstraction
> + *
> + * This is the abstraction layer for multithreads management which is
> + * used to speed up migration.
> + *
> + * Note: currently only one producer is allowed.
> + *
> + * Copyright(C) 2018 Tencent Corporation.
> + *
> + * Author:
> + *   Xiao Guangrong <address@hidden>
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2.1 or 
> later.
> + * See the COPYING.LIB file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"

I was told (more than once) that we should not include "osdep.h" in
headers. :) I'll suggest you include that in the source file.

> +#include "hw/boards.h"

Why do we need this header?

> +
> +#include "ring.h"
> +
> +/*
> + * the request representation which contains the internally used mete data,
> + * it can be embedded to user's self-defined data struct and the user can
> + * use container_of() to get the self-defined data
> + */
> +struct ThreadRequest {
> +    QSLIST_ENTRY(ThreadRequest) node;
> +    unsigned int thread_index;
> +};
> +typedef struct ThreadRequest ThreadRequest;
> +
> +struct Threads;
> +
> +struct ThreadLocal {
> +    QemuThread thread;
> +
> +    /* the event used to wake up the thread */
> +    QemuEvent ev;
> +
> +    struct Threads *threads;
> +
> +    /* local request ring which is filled by the user */
> +    Ring *request_ring;
> +
> +    /* the index of the thread */
> +    int self;
> +
> +    /* thread is useless and needs to exit */
> +    bool quit;
> +};
> +typedef struct ThreadLocal ThreadLocal;
> +
> +/*
> + * the main data struct represents multithreads which is shared by
> + * all threads
> + */
> +struct Threads {
> +    const char *name;
> +    unsigned int threads_nr;
> +    /* the request is pushed to the thread with round-robin manner */
> +    unsigned int current_thread_index;
> +
> +    int thread_ring_size;
> +    int total_requests;
> +
> +    /* the request is pre-allocated and linked in the list */
> +    int free_requests_nr;
> +    QSLIST_HEAD(, ThreadRequest) free_requests;
> +
> +    /* the constructor of request */
> +    ThreadRequest *(*thread_request_init)(void);
> +    /* the destructor of request */
> +    void (*thread_request_uninit)(ThreadRequest *request);
> +    /* the handler of the request which is called in the thread */
> +    void (*thread_request_handler)(ThreadRequest *request);
> +    /*
> +     * the handler to process the result which is called in the
> +     * user's context
> +     */
> +    void (*thread_request_done)(ThreadRequest *request);
> +
> +    /* the thread push the result to this ring so it has multiple producers 
> */
> +    Ring *request_done_ring;
> +
> +    ThreadLocal per_thread_data[0];
> +};
> +typedef struct Threads Threads;

Not sure whether we can move Threads/ThreadLocal definition into the
source file, then we only expose the struct definition, along with the
APIs.

Regards,

> +
> +Threads *threads_create(unsigned int threads_nr, const char *name,
> +                        ThreadRequest *(*thread_request_init)(void),
> +                        void (*thread_request_uninit)(ThreadRequest 
> *request),
> +                        void (*thread_request_handler)(ThreadRequest 
> *request),
> +                        void (*thread_request_done)(ThreadRequest *request));
> +void threads_destroy(Threads *threads);
> +
> +/*
> + * find a free request and associate it with a free thread.
> + * If no request or no thread is free, return NULL
> + */
> +ThreadRequest *threads_submit_request_prepare(Threads *threads);
> +/*
> + * push the request to its thread's local ring and notify the thread
> + */
> +void threads_submit_request_commit(Threads *threads, ThreadRequest *request);
> +
> +/*
> + * wait all threads to complete the request filled in their local rings
> + * to make sure there is no previous request exists.
> + */
> +void threads_wait_done(Threads *threads);
> +#endif
> -- 
> 2.14.4
> 

-- 
Peter Xu