Re: [Qemu-devel] [PATCH for 2.6 1/3] backup: Use Bitmap to replace "s->bitmap"

[Wen Congyang <wency@cn.fujitsu.com>]

On 11/23/2015 05:55 PM, Fam Zheng wrote:
> On Mon, 11/23 17:24, Wen Congyang wrote:
>> On 11/23/2015 05:19 PM, Fam Zheng wrote:
>>> On Mon, 11/23 17:01, Wen Congyang wrote:
>>>> On 11/20/2015 05:59 PM, Fam Zheng wrote:
>>>>> "s->bitmap" tracks done sectors, we only check bit states without using any
>>>>> iterator which HBitmap is good for. Switch to "Bitmap" which is simpler and
>>>>> more memory efficient.
>>>>>
>>>>> Meanwhile, rename it to done_bitmap, to reflect the intention.
>>>>>
>>>>> Signed-off-by: Fam Zheng <famz@redhat.com>
>>>>> ---
>>>>>  block/backup.c | 11 ++++++-----
>>>>>  1 file changed, 6 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/block/backup.c b/block/backup.c
>>>>> index 3b39119..d408f98 100644
>>>>> --- a/block/backup.c
>>>>> +++ b/block/backup.c
>>>>> @@ -22,6 +22,7 @@
>>>>>  #include "qapi/qmp/qerror.h"
>>>>>  #include "qemu/ratelimit.h"
>>>>>  #include "sysemu/block-backend.h"
>>>>> +#include "qemu/bitmap.h"
>>>>>  
>>>>>  #define BACKUP_CLUSTER_BITS 16
>>>>>  #define BACKUP_CLUSTER_SIZE (1 << BACKUP_CLUSTER_BITS)
>>>>> @@ -47,7 +48,7 @@ typedef struct BackupBlockJob {
>>>>>      BlockdevOnError on_target_error;
>>>>>      CoRwlock flush_rwlock;
>>>>>      uint64_t sectors_read;
>>>>> -    HBitmap *bitmap;
>>>>> +    unsigned long *done_bitmap;
>>>>>      QLIST_HEAD(, CowRequest) inflight_reqs;
>>>>>  } BackupBlockJob;
>>>>>  
>>>>> @@ -113,7 +114,7 @@ static int coroutine_fn backup_do_cow(BlockDriverState *bs,
>>>>>      cow_request_begin(&cow_request, job, start, end);
>>>>>  
>>>>>      for (; start < end; start++) {
>>>>> -        if (hbitmap_get(job->bitmap, start)) {
>>>>> +        if (test_bit(start, job->done_bitmap)) {
>>>>>              trace_backup_do_cow_skip(job, start);
>>>>>              continue; /* already copied */
>>>>>          }
>>>>> @@ -164,7 +165,7 @@ static int coroutine_fn backup_do_cow(BlockDriverState *bs,
>>>>>              goto out;
>>>>>          }
>>>>>  
>>>>> -        hbitmap_set(job->bitmap, start, 1);
>>>>> +        bitmap_set(job->done_bitmap, start, 1);
>>>>
>>>> You can use set_bit() here.
>>>
>>> Why? I think bitmap_set is a better match with bitmap_new below.
>>
>> set_bit() is quicker than bitmap_set() if you only set one bit.
>>
> 
> How much quicker is it? This doesn't sound convincing enough for me to lose the
> readability.

I don't test it. It is just a suggestion.

Thanks
Wen Congyang

> 
> Fam
> .
>