Re: [PATCH] Revert "bfq: Fix computation of shallow depth"

[Paolo Valente]

> Il giorno 1 feb 2021, alle ore 08:32, Lin Feng <linf@xxxxxxxxxx> ha scritto:
> 
> Hi, it seems that this patch was blocked by linux mailist servers, so ping again.
> 
> Based on https://patchwork.kernel.org/project/linux-block/patch/20201210094433.25491-1-jack@xxxxxxx/,
> it looks like we have made a consensus about bfqd->word_depths[2][2]'s changing, so now the
> computation codes for bfq's word_depths array are not necessary and one variable is enough.
> 
> But IMHO async depth limitation for slow drivers is essential, which is what we always did in cfq age.
> 

It is essential.

Thanks,
Paolo

> On 1/29/21 19:18, Lin Feng wrote:
>> 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%?
>> Please correct me if I miss something, thanks.
>> Signed-off-by: Lin Feng <linf@xxxxxxxxxx>
>> ---
>>  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++)
>