Re: [PATCH RESEND 4/4] qemu-xen-dir/hw/block: Cache local buffers used in grant copy

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, May 31, 2016 at 06:44:58AM +0200, Paulina Szubarczyk wrote:
> If there are still pending requests the buffers are not free() but
> cached in an array of a size max_request*BLKIF_MAX_SEGMENTS_PER_REQUEST
> 
> ---
>  hw/block/xen_disk.c | 60 +++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 47 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
> index 43cd9c9..cf80897 100644
> --- a/hw/block/xen_disk.c
> +++ b/hw/block/xen_disk.c
> @@ -125,6 +125,10 @@ struct XenBlkDev {
>      /* */
>      gboolean            feature_discard;
>  
> +    /* request buffer cache */
> +    void                **buf_cache;
> +    int                 buf_cache_free;

Have you checked if there's some already available FIFO queue structure that 
you can use?

Glib Trash Stacks looks like a suitable candidate:

https://developer.gnome.org/glib/stable/glib-Trash-Stacks.html

> +
>      /* qemu block driver */
>      DriveInfo           *dinfo;
>      BlockBackend        *blk;
> @@ -284,12 +288,16 @@ err:
>      return -1;
>  }
>  
> -
> -static void* get_buffer(void) {
> +static void* get_buffer(struct XenBlkDev *blkdev) {
>      void *buf;
>  
> -    buf = mmap(NULL, 1 << XC_PAGE_SHIFT, PROT_READ | PROT_WRITE, 
> +    if(blkdev->buf_cache_free <= 0) {
> +        buf = mmap(NULL, 1 << XC_PAGE_SHIFT, PROT_READ | PROT_WRITE, 
>                 MAP_SHARED | MAP_ANONYMOUS, -1, 0);
> +    } else {
> +        blkdev->buf_cache_free--;
> +        buf = blkdev->buf_cache[blkdev->buf_cache_free];
> +    }
>  
>      if (unlikely(buf == MAP_FAILED))
>          return NULL;
> @@ -301,21 +309,40 @@ static int free_buffer(void* buf) {
>      return munmap(buf, 1 << XC_PAGE_SHIFT);
>  }
>  
> -static int free_buffers(void** page, int count) 
> +static int free_buffers(void** page, int count, struct XenBlkDev *blkdev) 
>  {
> -    int i, r = 0;
> +    int i, put_buf_cache = 0, r = 0;
> +
> +    if (blkdev->more_work && blkdev->requests_inflight < max_requests) {

Shouldn't this be <=?

Or else you will only cache at most 341 pages instead of the maximum 
number of pages that can be in-flight (352).

Roger.

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