On 4/5/11 9:39 AM, Eric Sandeen wrote:
> On 4/5/11 1:10 AM, Andreas Dilger wrote:
>> On 2011-04-04, at 9:11 AM, Eric Sandeen wrote:
>>> Block devices may set minimum or optimal IO hints equal to
>>> blocksize; in this case there is really nothing for ext4
>>> to do with this information (i.e. search for a block-aligned
>>> allocation?) so don't set fs geometry with single-block
>>> values.
>>>
>>> Zeev also reported that with a block-sized stripe, the
>>> ext4 allocator spends time spinning in ext4_mb_scan_aligned(),
>>> oddly enough.
>>>
>>> Reported-by: Zeev Tarantov <zeev.tarantov@xxxxxxxxx>
>>> Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
>>> ---
>>>
>>> diff --git a/misc/mke2fs.c b/misc/mke2fs.c
>>> index 9798b88..74b838c 100644
>>> --- a/misc/mke2fs.c
>>> +++ b/misc/mke2fs.c
>>> @@ -1135,8 +1135,11 @@ static int get_device_geometry(const char *file,
>>> if ((opt_io == 0) && (psector_size > blocksize))
>>> opt_io = psector_size;
>>>
>>> - fs_param->s_raid_stride = min_io / blocksize;
>>> - fs_param->s_raid_stripe_width = opt_io / blocksize;
>>> + /* setting stripe/stride to blocksize is pointless */
>>> + if (min_io > blocksize)
>>> + fs_param->s_raid_stride = min_io / blocksize;
>>> + if (opt_io > blocksize)
>>> + fs_param->s_raid_stripe_width = opt_io / blocksize;
>>
>> I don't think it is harmful to specify an mballoc alignment that is
>> an even multiple of the underlying device IO size (e.g. at least
>> 256kB or 512kB).
>>
>> If the underlying device (e.g. zram) is reporting 16kB or 64kB opt_io
>> size because that is PAGE_SIZE, but blocksize is 4kB, then we will
>> have the same performance problem again.>
>> Cheers, Andreas
>
> I need to look into why ext4_mb_scan_aligned is so inefficient for a block-sized stripe.
>
> In practice I don't think we've seen this problem with stripe size at 4 or 8 or 16 blocks; it may just be less apparent. I think the function steps through by stripe-sized units, and if that is 1 block, it's a lot of stepping.
>
> while (i < EXT4_BLOCKS_PER_GROUP(sb)) {
> ...
> if (!mb_test_bit(i, bitmap)) {
Offhand I think maybe mb_find_next_zero_bit would be more efficient.
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1939,16 +1939,14 @@ void ext4_mb_scan_aligned(struct ext4_allocation_context *ac,
i = (a * sbi->s_stripe) - first_group_block;
while (i < EXT4_BLOCKS_PER_GROUP(sb)) {
- if (!mb_test_bit(i, bitmap)) {
- max = mb_find_extent(e4b, 0, i, sbi->s_stripe, &ex);
- if (max >= sbi->s_stripe) {
- ac->ac_found++;
- ac->ac_b_ex = ex;
- ext4_mb_use_best_found(ac, e4b);
- break;
- }
+ i = mb_find_next_zero_bit(bitmap, EXT4_BLOCKS_PER_GROUP(sb), i);
+ max = mb_find_extent(e4b, 0, i, sbi->s_stripe, &ex);
+ if (max >= sbi->s_stripe) {
+ ac->ac_found++;
+ ac->ac_b_ex = ex;
+ ext4_mb_use_best_found(ac, e4b);
+ break;
}
- i += sbi->s_stripe;
}
}
totally untested, but I think we have better ways to step through the bitmap.
-Eric
> ...
> }
> i += sbi->s_stripe;
> }
>
> But in any case, setting stripe alignment to 1 block makes no sense to me, and I see no reason to do it at mkfs time...
>
> -Eric
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