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

[Michael S. Tsirkin]

On Wed, Jul 30, 2014 at 04:14:09PM +0200, Paolo Bonzini wrote:
> 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. :(

BTW, this needs to be fixed anyway.

> 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