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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html