Re: [Qemu-devel] [PATCH v4 02/23] block: Add flag to avoid wasted work in bdrv_is_allocated()

[Eric Blake <eblake@redhat.com>]

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On 09/26/2017 01:31 PM, John Snow wrote:
> 
> 
> On 09/13/2017 12:03 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 <eblake@redhat.com>
>>

>>   *
>> + * If 'mapping' is true, the caller is querying for mapping purposes,
>> + * and the result should include BDRV_BLOCK_OFFSET_VALID where
>> + * possible; otherwise, the result may omit that bit particularly if
>> + * it allows for a larger value in 'pnum'.

I decided one more tweak to the comment will help:

+ * If 'mapping' is true, the caller is querying for mapping purposes,
+ * and the result should include BDRV_BLOCK_OFFSET_VALID and
+ * BDRV_BLOCK_ZERO where possible; otherwise, the result may omit those
+ * bits particularly if it allows for a larger value in 'pnum'.


>> @@ -1836,12 +1844,13 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
>>          }
>>      }
>>
>> -    if (local_file && local_file != bs &&
>> +    if (mapping && local_file && local_file != bs &&
> 
> Tentatively this looks OK to me, but I have to admit I'm a little shaky
> on this portion because I've not really investigated this function too
> much. I am at the very least convinced that when mapping is true that
> the function is equivalent and that existing callers don't have their
> behavior changed too much.
> 
> Benefit of the doubt:
> 
> Reviewed-by: John Snow <jsnow@redhat.com>

Then I'll tentatively keep your R-b even with the comment tweak, unless
you say otherwise :)

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


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