From: Eric Sandeen Subject: Re: [PATCH] e2fsprogs: don't set stripe/stride to 1 block in mkfs Date: Tue, 05 Apr 2011 09:56:06 -0700 Message-ID: <4D9B49A6.7000709@redhat.com> References: <4D9A17F8.4000406@redhat.com> <4D9B45AB.8000208@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: ext4 development , Zeev Tarantov To: Andreas Dilger Return-path: Received: from mx1.redhat.com ([209.132.183.28]:1031 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753849Ab1DEQ4L (ORCPT ); Tue, 5 Apr 2011 12:56:11 -0400 In-Reply-To: <4D9B45AB.8000208@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 >>> Signed-off-by: Eric Sandeen >>> --- >>> >>> 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html