On 4/5/11 9:56 AM, Eric Sandeen wrote:
> 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.
I tested it ;)
Seems to work fine, though I probably should see how things actually got allocated.
Creating an fs with 1-block stripes & widths as with the original report, and copying a (built) 2.6 linux kernel tree, it took about 7 minutes, and looped in the while() loop above 328215171 times.
With the patch above, it took 6m30s, and looped 25055 times.
For a filesystem with no stripe/stride set, it took 6m26s.
Hm, but subsequent tests w/ the tiny stripe set came around 6m30s as well. So there's no obvious speedup.
Still, avoiding all that looping seems beneficial, I can send a patch after I make sure allocation is still happening as expected.
Zeev, if you'd like to test that patch above with your profiling, that'd be awesome.
Thanks,
-Eric
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