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Re: [Qemu-devel] [PATCH 3/3] qcow2: reorder fields in Qcow2CachedTable t
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH 3/3] qcow2: reorder fields in Qcow2CachedTable to reduce padding |
Date: |
Tue, 26 May 2015 10:12:51 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
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 <address@hidden>
>> ---
>> 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 <address@hidden>
>
>
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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[Qemu-devel] [PATCH 2/3] qcow2: add option to clean unused cache entries after some time, Alberto Garcia, 2015/05/18