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Re: [Qemu-block] [Qemu-devel] [PATCH v2 03/15] block: Add flag to avoid

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2 03/15] block: Add flag to avoid wasted work in bdrv_is_allocated()
Date: Wed, 5 Jul 2017 21:35:36 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 07/03/2017 05:14 PM, Eric Blake wrote:
> Not all callers care about which BDS owns the mapping for a given
> range of the file.  In particular, bdrv_is_allocated() cares more
> about finding the largest run of allocated data from the guest
> perspective, whether or not that data is consecutive from the
> host perspective.  Therefore, doing subsequent refinements such
> as checking how much of the format-layer allocation also satisfies
> BDRV_BLOCK_ZERO at the protocol layer is wasted work - in the best
> case, it just costs extra CPU cycles during a single
> bdrv_is_allocated(), but in the worst case, it results in a smaller
> *pnum, and forces callers to iterate through more status probes when
> visiting the entire file for even more extra CPU cycles.
> This patch only optimizes the block layer.  But subsequent patches
> will tweak the driver callback to be byte-based, and in the process,
> can also pass this hint through to the driver.
> Signed-off-by: Eric Blake <address@hidden>

> @@ -1810,12 +1817,13 @@ static int64_t coroutine_fn 
> bdrv_co_get_block_status(BlockDriverState *bs,
>          }
>      }
> -    if (local_file && local_file != bs &&
> +    if (!allocation && local_file && local_file != bs &&
>          (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
>          (ret & BDRV_BLOCK_OFFSET_VALID)) {
>          int file_pnum;
> -        ret2 = bdrv_co_get_block_status(local_file, ret >> BDRV_SECTOR_BITS,
> +        ret2 = bdrv_co_get_block_status(local_file, true,
> +                                        ret >> BDRV_SECTOR_BITS,
>                                          *pnum, &file_pnum, NULL);
>          if (ret2 >= 0) {
>              /* Ignore errors.  This is just providing extra information, it

Hmm. My initial thinking here was that if we already have a good primary
status, we want our secondary status (where we are probing bs->file for
whether we can add BDRV_BLOCK_ZERO) to be as fast as possible, so I
hard-coded the answer that favors is_allocated (I have to be careful
describing this, since v3 will switch from 'bool allocated=true' to
'bool mapping=false' to express that same request).  But it turns out
that, at least for file-posix.c (for that matter, for several protocol
drivers), it's a LOT faster to just blindly state that the entire file
is allocated and data than it is to lseek(SEEK_HOLE).  So favoring
allocation status instead of mapping status defeats the purpose, and
this should be s/true/allocation/ (which is always false at this point)
[or conversely s/false/mapping/, which is always true, in v3].

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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