Re: [Qemu-devel] [PATCH 3/3] qcow2: reorder fields in Qcow2CachedTable to reduce padding

[Eric Blake <eblake@redhat.com>]

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On 05/26/2015 10:10 AM, Max Reitz wrote:
> On 18.05.2015 18:48, Alberto Garcia wrote:
>> Changing the current ordering saves 8 bytes per cache entry in x86_64.
> 
> Hm, not seven?
> 
>> Signed-off-by: Alberto Garcia <berto@igalia.com>
>> ---
>>   block/qcow2-cache.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
>> index a215f5b..43590ff 100644
>> --- a/block/qcow2-cache.c
>> +++ b/block/qcow2-cache.c
>> @@ -31,9 +31,9 @@
>>     typedef struct Qcow2CachedTable {
>>       int64_t  offset;
>> -    bool     dirty;
>>       uint64_t lru_counter;
>>       int      ref;
>> +    bool     dirty;
>>   } Qcow2CachedTable;
>>     struct Qcow2Cache {
> 
> With "7" above, or an explanation why it actually is 8:

Old layout:

0-7   offset
8     dirty
9-15  padding
16-23 lru_counter
24-27 dirty
28-31 padding

New layout:

0-7   offset
8-15  lru_counter
16-19 ref
20    dirty
21-23 padding

It indeed saves 8 bytes.

> 
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> 
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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