Re: [PATCH] Revert "bfq: Fix computation of shallow depth"
From: Paolo Valente <hidden>
Date: 2021-02-01 19:03:49
Also in:
lkml
Il giorno 29 gen 2021, alle ore 12:18, Lin Feng [off-list ref] ha scritto:
This reverts commit 6d4d273588378c65915acaf7b2ee74e9dd9c130a.
bfq.limit_depth passes word_depths[] as shallow_depth down to sbitmap core
sbitmap_get_shallow, which uses just the number to limit the scan depth of
each bitmap word, formula:
scan_percentage_for_each_word = shallow_depth / (1 << sbimap->shift) * 100%
That means the comments's percentiles 50%, 75%, 18%, 37% of bfq are correct.
But after commit patch 'bfq: Fix computation of shallow depth', we use
sbitmap.depth instead, as a example in following case:
sbitmap.depth = 256, map_nr = 4, shift = 6; sbitmap_word.depth = 64.
The resulsts of computed bfqd->word_depths[] are {128, 192, 48, 96}, and
three of the numbers exceed core dirver's 'sbitmap_word.depth=64' limit
nothing. Do we really don't want limit depth for such workloads, or we
just want to bump up the percentiles to 100%?Bumping to 100% would be a mistake. Thanks, Paolo
quoted hunk
Please correct me if I miss something, thanks. Signed-off-by: Lin Feng <redacted> --- block/bfq-iosched.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 9e4eb0fc1c16..9e81d1052091 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c@@ -6332,13 +6332,13 @@ static unsigned int bfq_update_depths(struct bfq_data *bfqd,* limit 'something'. */ /* no more than 50% of tags for async I/O */ - bfqd->word_depths[0][0] = max(bt->sb.depth >> 1, 1U); + bfqd->word_depths[0][0] = max((1U << bt->sb.shift) >> 1, 1U); /* * no more than 75% of tags for sync writes (25% extra tags * w.r.t. async I/O, to prevent async I/O from starving sync * writes) */ - bfqd->word_depths[0][1] = max((bt->sb.depth * 3) >> 2, 1U); + bfqd->word_depths[0][1] = max(((1U << bt->sb.shift) * 3) >> 2, 1U); /* * In-word depths in case some bfq_queue is being weight-@@ -6348,9 +6348,9 @@ static unsigned int bfq_update_depths(struct bfq_data *bfqd,* shortage. */ /* no more than ~18% of tags for async I/O */ - bfqd->word_depths[1][0] = max((bt->sb.depth * 3) >> 4, 1U); + bfqd->word_depths[1][0] = max(((1U << bt->sb.shift) * 3) >> 4, 1U); /* no more than ~37% of tags for sync writes (~20% extra tags) */ - bfqd->word_depths[1][1] = max((bt->sb.depth * 6) >> 4, 1U); + bfqd->word_depths[1][1] = max(((1U << bt->sb.shift) * 6) >> 4, 1U); for (i = 0; i < 2; i++) for (j = 0; j < 2; j++) -- 2.25.4