[f2fs-dev] [RFC PATCH 1/1] f2fs: adjust readahead window based on extent length

[f2fs-dev] [RFC PATCH 1/1] f2fs: adjust readahead window based on extent length

[Yongpeng Yang]

From: Yongpeng Yang <yangyongpeng@xiaomi.com>

For readahead, if the current readahead window is smaller than the extent
size, expand the window so that larger bios can be issued and improve
overall read performance.

Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
---
 fs/f2fs/data.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index c30e69392a62..2e5cbdb7729c 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -2377,6 +2377,17 @@ static int f2fs_mpage_readpages(struct inode *inode,
 	map.m_seg_type = NO_CHECK_TYPE;
 	map.m_may_create = false;
 
+	if (rac) {
+		loff_t block_in_file;
+
+		block_in_file = rac->_index;
+		map.m_lblk = block_in_file;
+		map.m_len = max(nr_pages, inode_to_bdi(inode)->ra_pages);
+		if (!f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_DEFAULT) && map.m_len > nr_pages) {
+			readahead_expand(rac, block_in_file << F2FS_BLKSIZE_BITS, map.m_len << F2FS_BLKSIZE_BITS);
+			nr_pages = readahead_count(rac);
+		}
+	}
 	for (; nr_pages; nr_pages--) {
 		if (rac) {
 			folio = readahead_folio(rac);
-- 
2.43.0



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Re: [f2fs-dev] [RFC PATCH 1/1] f2fs: adjust readahead window based on extent length

[Chao Yu via Linux-f2fs-devel]

On 12/12/25 21:52, Yongpeng Yang wrote:
> From: Yongpeng Yang <yangyongpeng@xiaomi.com>
> 
> For readahead, if the current readahead window is smaller than the extent
> size, expand the window so that larger bios can be issued and improve
> overall read performance.

Yongpeng,

Do you have any numbers to reveal how this patch improves the sequential
read performance?

In addition, will we suffer regression for random read case?

Thanks,

> 
> Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
> ---
>  fs/f2fs/data.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index c30e69392a62..2e5cbdb7729c 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -2377,6 +2377,17 @@ static int f2fs_mpage_readpages(struct inode *inode,
>  	map.m_seg_type = NO_CHECK_TYPE;
>  	map.m_may_create = false;
>  
> +	if (rac) {
> +		loff_t block_in_file;
> +
> +		block_in_file = rac->_index;
> +		map.m_lblk = block_in_file;
> +		map.m_len = max(nr_pages, inode_to_bdi(inode)->ra_pages);
> +		if (!f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_DEFAULT) && map.m_len > nr_pages) {
> +			readahead_expand(rac, block_in_file << F2FS_BLKSIZE_BITS, map.m_len << F2FS_BLKSIZE_BITS);
> +			nr_pages = readahead_count(rac);
> +		}
> +	}
>  	for (; nr_pages; nr_pages--) {
>  		if (rac) {
>  			folio = readahead_folio(rac);



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Re: [f2fs-dev] [RFC PATCH 1/1] f2fs: adjust readahead window based on extent length

[Jaegeuk Kim via Linux-f2fs-devel]

On 12/12, Yongpeng Yang wrote:
> From: Yongpeng Yang <yangyongpeng@xiaomi.com>
> 
> For readahead, if the current readahead window is smaller than the extent
> size, expand the window so that larger bios can be issued and improve
> overall read performance.

I don't think it's good to change the readahead size in deep filesystem level.

> 
> Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
> ---
>  fs/f2fs/data.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index c30e69392a62..2e5cbdb7729c 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -2377,6 +2377,17 @@ static int f2fs_mpage_readpages(struct inode *inode,
>  	map.m_seg_type = NO_CHECK_TYPE;
>  	map.m_may_create = false;
>  
> +	if (rac) {
> +		loff_t block_in_file;
> +
> +		block_in_file = rac->_index;
> +		map.m_lblk = block_in_file;
> +		map.m_len = max(nr_pages, inode_to_bdi(inode)->ra_pages);
> +		if (!f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_DEFAULT) && map.m_len > nr_pages) {
> +			readahead_expand(rac, block_in_file << F2FS_BLKSIZE_BITS, map.m_len << F2FS_BLKSIZE_BITS);
> +			nr_pages = readahead_count(rac);
> +		}
> +	}
>  	for (; nr_pages; nr_pages--) {
>  		if (rac) {
>  			folio = readahead_folio(rac);
> -- 
> 2.43.0


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Re: [f2fs-dev] [RFC PATCH 1/1] f2fs: adjust readahead window based on extent length

[Yongpeng Yang]

On 12/15/25 09:55, Chao Yu via Linux-f2fs-devel wrote:
> On 12/12/25 21:52, Yongpeng Yang wrote:
>> From: Yongpeng Yang <yangyongpeng@xiaomi.com>
>>
>> For readahead, if the current readahead window is smaller than the extent
>> size, expand the window so that larger bios can be issued and improve
>> overall read performance.
> 
> Yongpeng,
> 
> Do you have any numbers to reveal how this patch improves the sequential
> read performance?

Yes, the goal is to improve sequential read performance when the
readahead window is not aligned with the device optimal I/O size.

> 
> In addition, will we suffer regression for random read case?
> 

*Background*
On Android systems, some SoC vendors reduce the readahead window in
order to avoid excessive useless readahead. As a result, the readahead
window can be smaller than the device optimal I/O size (q->limits.io_opt).
When reading large files with dd, the number of read I/Os issued by
f2fs_readahead is limited by the readahead window, preventing the device
from fully utilizing its performance.

*Test setup*
1. Test environment: UFS 4.1 device, q->limits.io_opt is 1 MB
2. Test conditions: a 2 GB file with all extents sized at 2 MB
3. Test variables: the readahead window (read_ahead_kb) and the
effective readahead size after adjustment by readahead_expand in
f2fs_readahead
4. Test case: dd if=file_2m.dat of=/dev/null bs=512K count=4096, The
file page cache is dropped between runs, and the test is repeated 3 times.

*Test results*
| read_ahead_kb | effective f2fs_readahead window | bandwidth (GB/s)|
|---------------|---------------------------------|-----------------|
| 512 KB        | 512 KB                          | 2.2             |
| 512 KB        | 1024KB                          | 2.6             |
| 1024KB        | 1024KB                          | 2.6             |
| 1024KB        | 2048KB                          | 3.0             |

*Analysis*
1. The first row reflects the commonly used configuration and serves as
the baseline.
2. When the readahead window is aligned with the device optimal I/O
size, the bandwidth improves by about 18%.
3。 Increasing the effective f2fs_readahead window to 2 MB improves
bandwidth by about 36%.

*Disscusion*
The goal of this patch is to improve sequential read bandwidth when the
readahead window is smaller than the device optimal I/O size. Would it
make sense to record the end offset of the previous file access in
f2fs_read_ahead to determine whether the current read is sequential, and
based on that, decide whether to increase the readahead size?

Thanks
Yongpeng,

> Thanks,
> 
>>
>> Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
>> ---
>>  fs/f2fs/data.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index c30e69392a62..2e5cbdb7729c 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -2377,6 +2377,17 @@ static int f2fs_mpage_readpages(struct inode *inode,
>>  	map.m_seg_type = NO_CHECK_TYPE;
>>  	map.m_may_create = false;
>>  
>> +	if (rac) {
>> +		loff_t block_in_file;
>> +
>> +		block_in_file = rac->_index;
>> +		map.m_lblk = block_in_file;
>> +		map.m_len = max(nr_pages, inode_to_bdi(inode)->ra_pages);
>> +		if (!f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_DEFAULT) && map.m_len > nr_pages) {
>> +			readahead_expand(rac, block_in_file << F2FS_BLKSIZE_BITS, map.m_len << F2FS_BLKSIZE_BITS);
>> +			nr_pages = readahead_count(rac);
>> +		}
>> +	}
>>  	for (; nr_pages; nr_pages--) {
>>  		if (rac) {
>>  			folio = readahead_folio(rac);
> 
> 
> 
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> Linux-f2fs-devel@lists.sourceforge.net
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