Re: [PATCH 1/1] block: Check the queue limit before bio submitting

[Ed Tsai (蔡宗軒)]

On Sat, 2023-11-04 at 00:20 +0800, Ming Lei wrote:
>  On Wed, Nov 01, 2023 at 02:23:26AM +0000, Ed Tsai (蔡宗軒) wrote:
> > On Wed, 2023-10-25 at 17:22 +0800, ed.tsai@xxxxxxxxxxxx wrote:
> > > From: Ed Tsai <ed.tsai@xxxxxxxxxxxx>
> > > 
> > > Referring to commit 07173c3ec276 ("block: enable multipage
> bvecs"),
> > > each bio_vec now holds more than one page, potentially exceeding
> > > 1MB in size and causing alignment issues with the queue limit.
> > > 
> > > In a sequential read/write scenario, the file system maximizes
> the
> > > bio's capacity before submitting. However, misalignment with the
> > > queue limit can result in the bio being split into smaller I/O
> > > operations.
> > > 
> > > For instance, assuming the maximum I/O size is set to 512KB and
> the
> > > memory is highly fragmented, resulting in each bio containing
> only
> > > one 2-pages bio_vec (i.e., bi_size = 1028KB). This would cause
> the
> > > bio to be split into two 512KB portions and one 4KB portion. As a
> > > result, the originally expected continuous large I/O operations
> are
> > > interspersed with many small I/O operations.
> > > 
> > > To address this issue, this patch adds a check for the
> max_sectors
> > > before submitting the bio. This allows the upper layers to
> > > proactively
> > > detect and handle alignment issues.
> > > 
> > > I performed the Antutu V10 Storage Test on a UFS 4.0 device,
> which
> > > resulted in a significant improvement in the Sequential test:
> > > 
> > > Sequential Read (average of 5 rounds):
> > > Original: 3033.7 MB/sec
> > > Patched: 3520.9 MB/sec
> > > 
> > > Sequential Write (average of 5 rounds):
> > > Original: 2225.4 MB/sec
> > > Patched: 2800.3 MB/sec
> > > 
> > > Signed-off-by: Ed Tsai <ed.tsai@xxxxxxxxxxxx>
> > > ---
> > >  block/bio.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/block/bio.c b/block/bio.c
> > > index 816d412c06e9..a4a1f775b9ea 100644
> > > --- a/block/bio.c
> > > +++ b/block/bio.c
> > > @@ -1227,6 +1227,7 @@ static int __bio_iov_iter_get_pages(struct
> bio
> > > *bio, struct iov_iter *iter)
> > >  iov_iter_extraction_t extraction_flags = 0;
> > >  unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
> > >  unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
> > > +struct queue_limits *lim = &bdev_get_queue(bio->bi_bdev)-
> > > >limits;
> > >  struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
> > >  struct page **pages = (struct page **)bv;
> > >  ssize_t size, left;
> > > @@ -1275,6 +1276,11 @@ static int __bio_iov_iter_get_pages(struct
> bio
> > > *bio, struct iov_iter *iter)
> > >  struct page *page = pages[i];
> > >  
> > >  len = min_t(size_t, PAGE_SIZE - offset, left);
> > > +if (bio->bi_iter.bi_size + len >
> > > +    lim->max_sectors << SECTOR_SHIFT) {
> > > +ret = left;
> > > +break;
> > > +}
> > >  if (bio_op(bio) == REQ_OP_ZONE_APPEND) {
> > >  ret = bio_iov_add_zone_append_page(bio, page,
> > > len,
> > >  offset);
> > > -- 
> > > 2.18.0
> > > 
> > 
> > 
> > Hi Jens,
> > 
> > Just to clarify any potential confusion, I would like to provide
> > further details based on the assumed scenario mentioned above.
> > 
> > When the upper layer continuously sends 1028KB full-sized bios for
> > sequential reads, the Block Layer sees the following sequence:
> > submit bio: size = 1028KB, start LBA = n
> > submit bio: size = 1028KB, start LBA = n + 1028KB 
> > submit bio: size = 1028KB, start LBA = n + 2056KB
> > ...
> > 
> > However, due to the queue limit restricting the I/O size to a
> maximum
> > of 512KB, the Block Layer splits into the following sequence:
> > submit bio: size = 512KB, start LBA = n
> > submit bio: size = 512KB, start LBA = n +  512KB
> > submit bio: size =   4KB, start LBA = n + 1024KB
> > submit bio: size = 512KB, start LBA = n + 1028KB
> > submit bio: size = 512KB, start LBA = n + 1540KB
> > submit bio: size =   4KB, start LBA = n + 2052KB
> > submit bio: size = 512KB, start LBA = n + 2056KB
> > submit bio: size = 512KB, start LBA = n + 2568KB
> > submit bio: size =   4KB, start LBA = n + 3080KB
> > ...
> > 
> > The original expectation was for the storage to receive large,
> > contiguous requests. However, due to non-alignment, many small I/O
> > requests are generated. This problem is easily visible because the
> > user pages passed in are often allocated by the buddy system as
> order 0
> > pages during page faults, resulting in highly non-contiguous
> memory.
> 
> If order 0 page is added to bio, the multipage bvec becomes nop
> basically(256bvec holds 256 pages), then how can it make a difference
> for you?



> 
> > 
> > As observed in the Antutu Sequential Read test below, it is similar
> to
> > the description above where the splitting caused by the queue limit
> > leaves small requests sandwiched in between:
> > 
> > block_bio_queue: 8,32 R 86925864 + 2144 [Thread-51]
> > block_split: 8,32 R 86925864 / 86926888 [Thread-51]
> > block_split: 8,32 R 86926888 / 86927912 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86925864 + 1024 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86926888 + 1024 [Thread-51]
> > block_bio_queue: 8,32 R 86928008 + 2144 [Thread-51]
> > block_split: 8,32 R 86928008 / 86929032 [Thread-51]
> > block_split: 8,32 R 86929032 / 86930056 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86928008 + 1024 [Thread-51]
> > block_rq_issue: 8,32 R 49152 () 86927912 + 96 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86929032 + 1024 [Thread-51]
> > block_bio_queue: 8,32 R 86930152 + 2112 [Thread-51]
> > block_split: 8,32 R 86930152 / 86931176 [Thread-51]
> > block_split: 8,32 R 86931176 / 86932200 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86930152 + 1024 [Thread-51]
> > block_rq_issue: 8,32 R 49152 () 86930056 + 96 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86931176 + 1024 [Thread-51]
> > block_bio_queue: 8,32 R 86932264 + 2096 [Thread-51]
> > block_split: 8,32 R 86932264 / 86933288 [Thread-51]
> > block_split: 8,32 R 86933288 / 86934312 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86932264 + 1024 [Thread-51]
> > block_rq_issue: 8,32 R 32768 () 86932200 + 64 [Thread-51]
> > block_rq_issue: 8,32 R 524288 () 86933288 + 1024 [Thread-51]
> > 
> > I simply prevents non-aligned situations in bio_iov_iter_get_pages.
> 
> But there is still 4KB IO left if you limit max bio size is 512KB,
> then how does this 4KB IO finally go in case of 1028KB IO?
> 
> > Besides making the upper layer application aware of the queue
> limit, I
> > would appreciate any other directions or suggestions you may have.
> 
> The problem is related with IO size from application.
> 
> If you send unaligned IO, you can't avoid the last IO with small
> size, no
> matter if block layer bio split is involved or not. Your patch just
> lets
> __bio_iov_iter_get_pages split the bio, and you still have 4KB left
> finally when application submits 1028KB, right?
> 
> Then I don't understand why your patch improves sequential IO
> performance.
> 
> Thanks,
> Ming
> 

The application performs I/O with a sufficitenly large I/O size,
causing it to constantly fill up and submit full bios. However, in the
iomap direct I/O scenario, pages are added to the bio one by one from
the user buffer. This typically triggers page faults, resulting in the
allocation of order 0 pages from the buddy system.

The remaining amount of each order in the buddy system varies over
time. If there are not enough pages available in a particular order,
pages are split from higher orders. When pages are obtained from the
higher order, the user buffer may contain some small consecutive
patterns.

In summary, the physical layout of the user buffer is unpredictable,
and when it contains some small consecutive patterns, the size of the
bio becomes randomly unaligned during filling.

This patch limits the bio to be filled up to the max_sectors. The
submission is an async operation, so once the bio is queued, it will
immediately return and continue filled and submit the next bio.

Best,
Ed