Re: [Qemu-devel] [PATCH 07/15] dataplane: use object pool to speed up allocation for virtio blk request

[Paolo Bonzini]

Il 30/07/2014 13:39, Ming Lei ha scritto:
> g_slice_new(VirtIOBlockReq), its free pair and access the instance
> is a bit slow since sizeof(VirtIOBlockReq) takes more than 40KB,
> so use object pool to speed up its allocation and release.
> 
> With this patch, ~5% throughput improvement is observed in the VM
> based on server.
> 
> Signed-off-by: Ming Lei <address@hidden>
> ---
>  hw/block/dataplane/virtio-blk.c |   12 ++++++++++++
>  hw/block/virtio-blk.c           |   13 +++++++++++--
>  include/hw/virtio/virtio-blk.h  |    2 ++
>  3 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 2093e4a..828fe99 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -24,6 +24,8 @@
>  #include "hw/virtio/virtio-bus.h"
>  #include "qom/object_interfaces.h"
>  
> +#define REQ_POOL_SZ 128
> +
>  struct VirtIOBlockDataPlane {
>      bool started;
>      bool starting;
> @@ -51,6 +53,10 @@ struct VirtIOBlockDataPlane {
>      Error *blocker;
>      void (*saved_complete_request)(struct VirtIOBlockReq *req,
>                                     unsigned char status);
> +
> +    VirtIOBlockReq  reqs[REQ_POOL_SZ];
> +    void *free_reqs[REQ_POOL_SZ];
> +    ObjPool  req_pool;
>  };
>  
>  /* Raise an interrupt to signal guest, if necessary */
> @@ -238,6 +244,10 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
>          return;
>      }
>  
> +    vblk->obj_pool = &s->req_pool;
> +    obj_pool_init(vblk->obj_pool, s->reqs, s->free_reqs,
> +                  sizeof(VirtIOBlockReq), REQ_POOL_SZ);
> +
>      /* Set up guest notifier (irq) */
>      if (k->set_guest_notifiers(qbus->parent, 1, true) != 0) {
>          fprintf(stderr, "virtio-blk failed to set guest notifier, "
> @@ -298,6 +308,8 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
>  
>      aio_context_release(s->ctx);
>  
> +    vblk->obj_pool = NULL;
> +
>      if (s->raw_format) {
>          qemu_aio_set_bypass_co(s->ctx, false);
>      }
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index c241c50..2a11bc4 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -31,7 +31,11 @@
>  
>  VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
>  {
> -    VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);
> +    VirtIOBlockReq *req = obj_pool_get(s->obj_pool);
> +
> +    if (!req) {
> +        req = g_slice_new(VirtIOBlockReq);
> +    }
>      req->dev = s;
>      req->qiov.size = 0;
>      req->next = NULL;
> @@ -41,7 +45,11 @@ VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
>  void virtio_blk_free_request(VirtIOBlockReq *req)
>  {
>      if (req) {
> -        g_slice_free(VirtIOBlockReq, req);
> +        if (obj_pool_has_obj(req->dev->obj_pool, req)) {
> +            obj_pool_put(req->dev->obj_pool, req);
> +        } else {
> +            g_slice_free(VirtIOBlockReq, req);
> +        }
>      }
>  }
>  
> @@ -801,6 +809,7 @@ static void virtio_blk_instance_init(Object *obj)
>  {
>      VirtIOBlock *s = VIRTIO_BLK(obj);
>  
> +    s->obj_pool = NULL;
>      object_property_add_link(obj, "iothread", TYPE_IOTHREAD,
>                               (Object **)&s->blk.iothread,
>                               qdev_prop_allow_set_link_before_realize,
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index afb7b8d..49ac234 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -18,6 +18,7 @@
>  #include "hw/block/block.h"
>  #include "sysemu/iothread.h"
>  #include "block/block.h"
> +#include "qemu/obj_pool.h"
>  
>  #define TYPE_VIRTIO_BLK "virtio-blk-device"
>  #define VIRTIO_BLK(obj) \
> @@ -135,6 +136,7 @@ typedef struct VirtIOBlock {
>      Notifier migration_state_notifier;
>      struct VirtIOBlockDataPlane *dataplane;
>  #endif
> +    ObjPool *obj_pool;
>  } VirtIOBlock;
>  
>  typedef struct MultiReqBuffer {
> 

The problem is that g_slice here is not using the slab-style allocator
because the object is larger than roughly 500 bytes.  One solution would
be to make virtqueue_pop/vring_pop allocate a VirtQueueElement of the
right size (and virtqueue_push/vring_push free it), as mentioned in the
review of patch 8.

However, I now remembered that VirtQueueElement is a mess because it's
serialized directly into the migration state. :(  So you basically
cannot change it without mucking with migration.  Please leave out patch
8 for now.

Let's use this object pool, however, please simplify the API, it should
be just:

void obj_pool_init(ObjPool *pool, unsigned obj_size, unsigned cnt);
void *obj_pool_alloc(ObjPool *pool);
void obj_pool_free(ObjPool *pool, void *obj);
void obj_pool_destroy(ObjPool *pool);

All allocations of the objs buffer, and all logic like

+    VirtIOBlockReq *req = obj_pool_get(s->obj_pool);
+
+    if (!req) {
+        req = g_slice_new(VirtIOBlockReq);
+    }

+        if (obj_pool_has_obj(req->dev->obj_pool, req)) {
+            obj_pool_put(req->dev->obj_pool, req);
+        } else {
+            g_slice_free(VirtIOBlockReq, req);
+        }

should be part of the object pool, not its users.

Paolo