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Date: Tue, 26 May 2015 18:14:09 +0200
From: Max Reitz <mreitz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 3/3] qcow2: reorder fields in
 Qcow2CachedTable to reduce padding
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To: Eric Blake <eblake@redhat.com>, Alberto Garcia <berto@igalia.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-block@nongnu.org

On 26.05.2015 18:12, Eric Blake wrote:
> 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.

Oh, I forgot about the padding at the end. Thanks!

Max

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