Re: Re: [PATCH] bio: limit bio max size.

[Changheun Lee <nanich.lee@samsung.com>]

>On 2021/01/12 17:52, Changheun Lee wrote:
>> From: "Changheun Lee" <nanich.lee@samsung.com>
>> 
>> bio size can grow up to 4GB when muli-page bvec is enabled.
>> but sometimes it would lead to inefficient behaviors.
>> in case of large chunk direct I/O, - 64MB chunk read in user space -
>> all pages for 64MB would be merged to a bio structure if memory address is
>> continued phsycally. it makes some delay to submit until merge complete.
>> bio max size should be limited as a proper size.
>
>But merging physically contiguous pages into the same bvec + later automatic bio
>split on submit should give you better throughput for large IOs compared to
>having to issue a bio chain of smaller BIOs that are arbitrarily sized and will
>likely need splitting anyway (because of DMA boundaries etc).
>
>Do you have a specific case where you see higher performance with this patch
>applied ? On Intel, BIO_MAX_SIZE would be 1MB... That is arbitrary and too small
>considering that many hardware can execute larger IOs than that.
>

When I tested 32MB chunk read with O_DIRECT in android, all pages of 32MB
is merged into a bio structure.
And elapsed time to merge complete was about 2ms.
It means first bio-submit is after 2ms.
If bio size is limited with 1MB with this patch, first bio-submit is about
100us by bio_full operation.
It's not large delay and can't be observed with low speed device.
But it's needed to reduce merge delay for high speed device.
I improved 512MB sequential read performance from 1900MB/s to 2000MB/s
with this patch on android platform.
As you said, 1MB might be small for some device.
But method is needed to re-size, or select the bio max size.

>
>> 
>> Signed-off-by: Changheun Lee <nanich.lee@samsung.com>
>> ---
>>  block/bio.c         | 2 +-
>>  include/linux/bio.h | 3 ++-
>>  2 files changed, 3 insertions(+), 2 deletions(-)
>> 
>> diff --git a/block/bio.c b/block/bio.c
>> index 1f2cc1fbe283..dbe14d675f28 100644
>> --- a/block/bio.c
>> +++ b/block/bio.c
>> @@ -877,7 +877,7 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>>  		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
>>  
>>  		if (page_is_mergeable(bv, page, len, off, same_page)) {
>> -			if (bio->bi_iter.bi_size > UINT_MAX - len) {
>> +			if (bio->bi_iter.bi_size > BIO_MAX_SIZE - len) {
>>  				*same_page = false;
>>  				return false;
>>  			}
>> diff --git a/include/linux/bio.h b/include/linux/bio.h
>> index 1edda614f7ce..0f49b354b1f6 100644
>> --- a/include/linux/bio.h
>> +++ b/include/linux/bio.h
>> @@ -20,6 +20,7 @@
>>  #endif
>>  
>>  #define BIO_MAX_PAGES		256
>> +#define BIO_MAX_SIZE		(BIO_MAX_PAGES * PAGE_SIZE)
>>  
>>  #define bio_prio(bio)			(bio)->bi_ioprio
>>  #define bio_set_prio(bio, prio)		((bio)->bi_ioprio = prio)
>> @@ -113,7 +114,7 @@ static inline bool bio_full(struct bio *bio, unsigned len)
>>  	if (bio->bi_vcnt >= bio->bi_max_vecs)
>>  		return true;
>>  
>> -	if (bio->bi_iter.bi_size > UINT_MAX - len)
>> +	if (bio->bi_iter.bi_size > BIO_MAX_SIZE - len)
>>  		return true;
>>  
>>  	return false;
>> 
>
>
>-- 
>Damien Le Moal
>Western Digital Research