Re: [Qemu-devel] [PATCH 01/17] cow: make reads go at a decent speed

[Paolo Bonzini]

Il 04/07/2013 04:20, Fam Zheng ha scritto:
> On Wed, 07/03 16:34, Paolo Bonzini wrote:
>> Do not do two reads for each sector; load each sector of the bitmap
>> and use bitmap operations to process it.
>>
>> Writes are still dog slow!
>>
>> Signed-off-by: Paolo Bonzini <address@hidden>
>> ---
>>  block/cow.c | 54 ++++++++++++++++++++++++++++++++----------------------
>>  1 file changed, 32 insertions(+), 22 deletions(-)
>>
>> diff --git a/block/cow.c b/block/cow.c
>> index 1cc2e89..204451e 100644
>> --- a/block/cow.c
>> +++ b/block/cow.c
>> @@ -126,18 +126,31 @@ static inline int cow_set_bit(BlockDriverState *bs, 
>> int64_t bitnum)
>>      return 0;
>>  }
>>  
>> -static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
>> +#define BITS_PER_BITMAP_SECTOR (512 * 8)
>> +
>> +/* Cannot use bitmap.c on big-endian machines.  */
>> +static int cow_test_bit(int64_t bitnum, const uint8_t *bitmap)
>>  {
>> -    uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
>> -    uint8_t bitmap;
>> -    int ret;
>> +    return (bitmap[bitnum / 8] & (1 << (bitnum & 7))) != 0;
>> +}
>>  
>> -    ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
>> -    if (ret < 0) {
>> -       return ret;
>> +static int cow_find_streak(const uint8_t *bitmap, int value, int start, int 
>> nb_sectors)
> I think type bool is better for 'value' as you don't booleanize it.  And
> also int64_t for start?

start is always between 0 and BITS_PER_BITMAP_SECTOR.

"value" here is a bit value, so 0 or 1 rather than true or false.  I
prefer to keep it as "int", but it can be changed.

Paolo

>> +{
>> +    int streak_value = value ? 0xFF : 0;
>> +    int last = MIN(start + nb_sectors, BITS_PER_BITMAP_SECTOR);
>> +    int bitnum = start;
>> +    while (bitnum < last) {
>> +        if ((bitnum & 7) == 0 && bitmap[bitnum / 8] == streak_value) {
>> +            bitnum += 8;
>> +            continue;
>> +        }
>> +        if (cow_test_bit(bitnum, bitmap) == value) {
>> +            bitnum++;
>> +            continue;
>> +        }
>> +        break;
>>      }
>> -
>> -    return !!(bitmap & (1 << (bitnum % 8)));
>> +    return MIN(bitnum, last) - start;
>>  }
>>  
>>  /* Return true if first block has been changed (ie. current version is
>> @@ -146,23 +159,20 @@ static inline int is_bit_set(BlockDriverState *bs, 
>> int64_t bitnum)
>>  static int coroutine_fn cow_co_is_allocated(BlockDriverState *bs,
>>          int64_t sector_num, int nb_sectors, int *num_same)
>>  {
>> +    int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
>> +    uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
>> +    uint8_t bitmap[512];
>> +    int ret;
>>      int changed;
>>  
>> -    if (nb_sectors == 0) {
>> -    *num_same = nb_sectors;
>> -    return 0;
>> -    }
>> -
>> -    changed = is_bit_set(bs, sector_num);
>> -    if (changed < 0) {
>> -        return 0; /* XXX: how to return I/O errors? */
>> -    }
>> -
>> -    for (*num_same = 1; *num_same < nb_sectors; (*num_same)++) {
>> -    if (is_bit_set(bs, sector_num + *num_same) != changed)
>> -        break;
>> +    ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
>> +    if (ret < 0) {
>> +        return ret;
>>      }
>>  
>> +    bitnum &= BITS_PER_BITMAP_SECTOR - 1;
>> +    changed = cow_test_bit(bitnum, bitmap);
>> +    *num_same = cow_find_streak(bitmap, changed, bitnum, nb_sectors);
>>      return changed;
>>  }
>>  
>> -- 
>> 1.8.2.1
>>
>>
>>
>