Re: [Qemu-devel] [RFC PATCH 3/3] qemu-coroutine: use a ring per thread for the pool

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/11/2014 13:39, Peter Lieven wrote:
> Am 28.11.2014 um 13:26 schrieb Paolo Bonzini:
>>
>> On 28/11/2014 12:46, Peter Lieven wrote:
>>>> I get:
>>>> Run operation 40000000 iterations 9.883958 s, 4046K operations/s, 247ns per coroutine
>>> Ok, understood, it "steals" the whole pool, right? Isn't that bad if we have more
>>> than one thread in need of a lot of coroutines?
>> Overall the algorithm is expected to adapt.  The N threads contribute to
>> the global release pool, so the pool will fill up N times faster than if
>> you had only one thread.  There can be some variance, which is why the
>> maximum size of the pool is twice the threshold (and probably could be
>> tuned better).
>>
>> Benchmarks are needed on real I/O too, of course, especially with high
>> queue depth.
> 
> Yes, cool. The atomic operations are a bit tricky at the first glance ;-)
> 
> Question:
>  Why is the pool_size increment atomic and the set to zero not?

Because the set to zero is not a read-modify-write operation, so it is
always atomic.  It's just not sequentially-consistent (see
docs/atomics.txt for some info on what that means).

> Idea:
>  If the release_pool is full why not put the coroutine in the thread alloc_pool instead of throwing it away? :-)

Because you can only waste 64 coroutines per thread.  But numbers cannot
be sneezed at, so it's worth doing it as a separate patch.

> Run operation 40000000 iterations 9.057805 s, 4416K operations/s, 226ns per coroutine
> 
> diff --git a/qemu-coroutine.c b/qemu-coroutine.c
> index 6bee354..edea162 100644
> --- a/qemu-coroutine.c
> +++ b/qemu-coroutine.c
> @@ -25,8 +25,9 @@ enum {
>  
>  /** Free list to speed up creation */
>  static QSLIST_HEAD(, Coroutine) release_pool = QSLIST_HEAD_INITIALIZER(pool);
> -static unsigned int pool_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;
>  
>  /* The GPrivate is only used to invoke coroutine_pool_cleanup.  */
>  static void coroutine_pool_cleanup(void *value);
> @@ -39,12 +40,12 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
>      if (CONFIG_COROUTINE_POOL) {
>          co = QSLIST_FIRST(&alloc_pool);
>          if (!co) {
> -            if (pool_size > POOL_BATCH_SIZE) {
> -                /* This is not exact; there could be a little skew between pool_size
> +            if (release_pool_size > POOL_BATCH_SIZE) {
> +                /* This is not exact; there could be a little skew between release_pool_size
>                   * and the actual size of alloc_pool.  But it is just a heuristic,
>                   * it does not need to be perfect.
>                   */
> -                pool_size = 0;
> +                alloc_pool_size = atomic_fetch_and(&release_pool_size, 0);
>                  QSLIST_MOVE_ATOMIC(&alloc_pool, &release_pool);
>                  co = QSLIST_FIRST(&alloc_pool);
>  
> @@ -53,6 +54,8 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
>                   */
>                  g_private_set(&dummy_key, &dummy_key);
>              }
> +        } else {
> +            alloc_pool_size--;
>          }
>          if (co) {
>              QSLIST_REMOVE_HEAD(&alloc_pool, pool_next);
> @@ -71,10 +74,15 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
>  static void coroutine_delete(Coroutine *co)
>  {
>      if (CONFIG_COROUTINE_POOL) {
> -        if (pool_size < POOL_BATCH_SIZE * 2) {
> +        if (release_pool_size < POOL_BATCH_SIZE * 2) {
>              co->caller = NULL;
>              QSLIST_INSERT_HEAD_ATOMIC(&release_pool, co, pool_next);
> -            atomic_inc(&pool_size);
> +            atomic_inc(&release_pool_size);
> +            return;
> +        } else if (alloc_pool_size < POOL_BATCH_SIZE) {
> +            co->caller = NULL;
> +            QSLIST_INSERT_HEAD(&alloc_pool, co, pool_next);
> +            alloc_pool_size++;
>              return;
>          }
>      }
> 
> 
> Bug?:
>  The release_pool is not cleanup up on termination I think.

That's not necessary, it is global.

Paolo