Hello!
On Fri 29-01-21 19:18:08, 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%
Looking at sbitmap_get_shallow() again more carefully, I agree that I
misunderstood how shallow_depth argument gets used and the original code
was correct and I broke it. Thanks for spotting this!
What I didn't notice is that shallow_depth indeed gets used for each bitmap
word separately and not for bitmap as a whole. I'd say this could use some
more documentation but that's unrelated to your revert. So feel free to add:
Reviewed-by: Jan Kara <[email protected]>
to your patch. Thanks.
Honza
>
> 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 <[email protected]>
> ---
> 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
>
--
Jan Kara <[email protected]>
SUSE Labs, CR
On 2/2/21 5:28 AM, Jan Kara wrote:
> Hello!
>
> On Fri 29-01-21 19:18:08, 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%
>
> Looking at sbitmap_get_shallow() again more carefully, I agree that I
> misunderstood how shallow_depth argument gets used and the original code
> was correct and I broke it. Thanks for spotting this!
>
> What I didn't notice is that shallow_depth indeed gets used for each bitmap
> word separately and not for bitmap as a whole. I'd say this could use some
> more documentation but that's unrelated to your revert. So feel free to add:
>
> Reviewed-by: Jan Kara <[email protected]>
I don't have the original patch (neither directly nor in the archive), so
I had to hand-apply it. In any case, applied for 5.11, thanks.
--
Jens Axboe
Hi all,
On 2/2/21 22:20, Jens Axboe wrote:
> On 2/2/21 5:28 AM, Jan Kara wrote:
>> Hello!
>>
>> On Fri 29-01-21 19:18:08, 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%
>>
>> Looking at sbitmap_get_shallow() again more carefully, I agree that I
>> misunderstood how shallow_depth argument gets used and the original code
>> was correct and I broke it. Thanks for spotting this!
>>
>> What I didn't notice is that shallow_depth indeed gets used for each bitmap
>> word separately and not for bitmap as a whole. I'd say this could use some
>> more documentation but that's unrelated to your revert. So feel free to add:
>>
>> Reviewed-by: Jan Kara <[email protected]>
>
> I don't have the original patch (neither directly nor in the archive), so
> I had to hand-apply it. In any case, applied for 5.11, thanks.
>
Take a look at linux-block.git tree, the hand-applied commit for this patch
is broken, the following changing line is left out:
- 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);
Sorry for making troubles to you, I will resend this patch with tiny commit
log typo fix(sbimap -> sbitmap) and attaching Jan's Reviewed-by, also thanks
his time for reviewing.
Hope this time lkml server will not block my patch.
Thanks,
linfeng
On 2/2/21 7:36 PM, Lin Feng wrote:
> Hi all,
>
> On 2/2/21 22:20, Jens Axboe wrote:
>> On 2/2/21 5:28 AM, Jan Kara wrote:
>>> Hello!
>>>
>>> On Fri 29-01-21 19:18:08, 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%
>>>
>>> Looking at sbitmap_get_shallow() again more carefully, I agree that I
>>> misunderstood how shallow_depth argument gets used and the original code
>>> was correct and I broke it. Thanks for spotting this!
>>>
>>> What I didn't notice is that shallow_depth indeed gets used for each bitmap
>>> word separately and not for bitmap as a whole. I'd say this could use some
>>> more documentation but that's unrelated to your revert. So feel free to add:
>>>
>>> Reviewed-by: Jan Kara <[email protected]>
>>
>> I don't have the original patch (neither directly nor in the archive), so
>> I had to hand-apply it. In any case, applied for 5.11, thanks.
>>
>
> Take a look at linux-block.git tree, the hand-applied commit for this patch
> is broken, the following changing line is left out:
> - 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);
>
> Sorry for making troubles to you, I will resend this patch with tiny commit
> log typo fix(sbimap -> sbitmap) and attaching Jan's Reviewed-by, also thanks
> his time for reviewing.
>
> Hope this time lkml server will not block my patch.
Thanks for checking - just send me an incremental and I'll fold it in.
--
Jens Axboe