Re: [PATCH 1/1] util: adjust coroutine pool size to virtio block queue

[Stefan Hajnoczi]

On Thu, Jan 06, 2022 at 05:20:57PM +0900, Hiroki Narukawa wrote:

Phil, thanks for notifying me.

> Coroutine pool size was 64 from long ago, and the basis was organized in the 
> commit message in c740ad92.
> 
> At that time, virtio-blk queue-size and num-queue were not configuable, and 
> equivalent values were 128 and 1.
> 
> Coroutine pool size 64 was fine then.
> 
> Later queue-size and num-queue got configuable, and default values were 
> increased.
> 
> Coroutine pool with size 64 exhausts frequently with random disk IO in new 
> size, and slows down.
> 
> This commit adjusts coroutine pool size adaptively with new values.
> 
> This commit adds 64 by default, but now coroutine is not only for block 
> devices,
> 
> and is not too much burdon comparing with new default.
> 
> pool size of 128 * vCPUs.
> 
> Signed-off-by: Hiroki Narukawa <hnarukaw@yahoo-corp.jp>
> ---
>  hw/block/virtio-blk.c    |  3 +++
>  include/qemu/coroutine.h |  5 +++++
>  util/qemu-coroutine.c    | 15 +++++++++++----
>  3 files changed, 19 insertions(+), 4 deletions(-)

Have you measured with QEMU 6.1 or later? Commit
d7ddd0a1618a75b31dc308bb37365ce1da972154 ("linux-aio: limit the batch
size using `aio-max-batch` parameter") can hide this issue so it may not
be apparent in recent QEMU releases.

I like your approach better than what I tried recently (I ended up
dropping the patch from my queue because it doesn't handle coroutines
created in one thread and terminated in another thread correctly):
https://patchew.org/QEMU/20210913153524.1190696-1-stefanha@redhat.com/

> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index f139cd7cc9..726dbe14de 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -32,6 +32,7 @@
>  #include "hw/virtio/virtio-bus.h"
>  #include "migration/qemu-file-types.h"
>  #include "hw/virtio/virtio-access.h"
> +#include "qemu/coroutine.h"
>  
>  /* Config size before the discard support (hide associated config fields) */
>  #define VIRTIO_BLK_CFG_SIZE offsetof(struct virtio_blk_config, \
> @@ -1222,6 +1223,8 @@ static void virtio_blk_device_realize(DeviceState *dev, 
> Error **errp)
>      for (i = 0; i < conf->num_queues; i++) {
>          virtio_add_queue(vdev, conf->queue_size, virtio_blk_handle_output);
>      }
> +    qemu_coroutine_increase_pool_batch_size(conf->num_queues * 
> conf->queue_size
> +                                            / 2);

This over-provisions coroutine pools when IOThreads are configured,
because --device virtio-blk-pci,iothread=iothread2 will only submit I/O
requests in iothread2, for example. Other threads don't need to increase
their limit.

However, I think it's okay to use this inexact approach. It's still
better than the current hardcoded 64 coroutine pool size.

>      virtio_blk_data_plane_create(vdev, conf, &s->dataplane, &err);
>      if (err != NULL) {
>          error_propagate(errp, err);
> diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
> index 4829ff373d..e52ed76ab2 100644
> --- a/include/qemu/coroutine.h
> +++ b/include/qemu/coroutine.h
> @@ -331,6 +331,11 @@ void qemu_co_sleep_wake(QemuCoSleep *w);
>   */
>  void coroutine_fn yield_until_fd_readable(int fd);
>  
> +/**
> + * Increase coroutine pool size
> + */
> +void qemu_coroutine_increase_pool_batch_size(unsigned int 
> additional_pool_size);
> +
>  #include "qemu/lockable.h"
>  
>  #endif /* QEMU_COROUTINE_H */
> diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> index 38fb6d3084..080a1e0126 100644
> --- a/util/qemu-coroutine.c
> +++ b/util/qemu-coroutine.c
> @@ -20,12 +20,14 @@
>  #include "qemu/coroutine_int.h"
>  #include "block/aio.h"
>  
> +/** Initial batch size is 64, and is increased on demand */
>  enum {
> -    POOL_BATCH_SIZE = 64,
> +    POOL_INITIAL_BATCH_SIZE = 64,
>  };
>  
>  /** Free list to speed up creation */
>  static QSLIST_HEAD(, Coroutine) release_pool = QSLIST_HEAD_INITIALIZER(pool);
> +static unsigned int pool_batch_size = POOL_INITIAL_BATCH_SIZE;
>  static unsigned int release_pool_size;
>  static __thread QSLIST_HEAD(, Coroutine) alloc_pool = 
> QSLIST_HEAD_INITIALIZER(pool);
>  static __thread unsigned int alloc_pool_size;
> @@ -49,7 +51,7 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry, 
> void *opaque)
>      if (CONFIG_COROUTINE_POOL) {
>          co = QSLIST_FIRST(&alloc_pool);
>          if (!co) {
> -            if (release_pool_size > POOL_BATCH_SIZE) {
> +            if (release_pool_size > pool_batch_size) {
>                  /* Slow path; a good place to register the destructor, too.  
> */
>                  if (!coroutine_pool_cleanup_notifier.notify) {
>                      coroutine_pool_cleanup_notifier.notify = 
> coroutine_pool_cleanup;
> @@ -86,12 +88,12 @@ static void coroutine_delete(Coroutine *co)
>      co->caller = NULL;
>  
>      if (CONFIG_COROUTINE_POOL) {
> -        if (release_pool_size < POOL_BATCH_SIZE * 2) {
> +        if (release_pool_size < pool_batch_size * 2) {
>              QSLIST_INSERT_HEAD_ATOMIC(&release_pool, co, pool_next);
>              qatomic_inc(&release_pool_size);
>              return;
>          }
> -        if (alloc_pool_size < POOL_BATCH_SIZE) {
> +        if (alloc_pool_size < pool_batch_size) {
>              QSLIST_INSERT_HEAD(&alloc_pool, co, pool_next);
>              alloc_pool_size++;
>              return;
> @@ -202,3 +204,8 @@ AioContext *coroutine_fn 
> qemu_coroutine_get_aio_context(Coroutine *co)
>  {
>      return co->ctx;
>  }
> +
> +void qemu_coroutine_increase_pool_batch_size(unsigned int 
> additional_pool_size)
> +{
> +    qatomic_add(&pool_batch_size, additional_pool_size);

If atomic_add() is used to modify pool_batch_size then qatomic_read()
should be used for loads. At a minimum it serves as documentation that
this is an atomic variable.

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