diff mbox series

[03/13] bcache: inherit the optimal I/O size

Message ID 20200921080734.452759-4-hch@lst.de
State New
Headers show
Series [01/13] fs: remove the unused SB_I_MULTIROOT flag | expand

Commit Message

Christoph Hellwig Sept. 21, 2020, 8:07 a.m. UTC
Inherit the optimal I/O size setting just like the readahead window,
as any reason to do larger I/O does not apply to just readahead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/md/bcache/super.c | 2 ++
 1 file changed, 2 insertions(+)

Comments

Coly Li Sept. 21, 2020, 9:54 a.m. UTC | #1
On 2020/9/21 16:07, Christoph Hellwig wrote:
> Inherit the optimal I/O size setting just like the readahead window,
> as any reason to do larger I/O does not apply to just readahead.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  drivers/md/bcache/super.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 1bbdc410ee3c51..48113005ed86ad 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1430,6 +1430,8 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
>  	dc->disk.disk->queue->backing_dev_info->ra_pages =
>  		max(dc->disk.disk->queue->backing_dev_info->ra_pages,
>  		    q->backing_dev_info->ra_pages);
> +	blk_queue_io_opt(dc->disk.disk->queue,
> +		max(queue_io_opt(dc->disk.disk->queue), queue_io_opt(q)));
>  

Hi Christoph,

I am not sure whether virtual bcache device's optimal request size can
be simply set like this.

Most of time inherit backing device's optimal request size is fine, but
there are two exceptions,
- Read request hits on cache device
- User sets sequential_cuttoff as 0, all writing may go into cache
device firstly.
For the above two conditions, all I/Os goes into cache device, using
optimal request size of backing device might be improper.

Just a guess, is it OK to set the optimal request size of the virtual
bcache device as the least common multiple of cache device's and backing
device's optimal request sizes ?


[snipped]

Thanks.

Coly Li
Christoph Hellwig Sept. 21, 2020, 2 p.m. UTC | #2
On Mon, Sep 21, 2020 at 05:54:59PM +0800, Coly Li wrote:
> I am not sure whether virtual bcache device's optimal request size can
> be simply set like this.
> 
> Most of time inherit backing device's optimal request size is fine, but
> there are two exceptions,
> - Read request hits on cache device
> - User sets sequential_cuttoff as 0, all writing may go into cache
> device firstly.
> For the above two conditions, all I/Os goes into cache device, using
> optimal request size of backing device might be improper.
> 
> Just a guess, is it OK to set the optimal request size of the virtual
> bcache device as the least common multiple of cache device's and backing
> device's optimal request sizes ?

Well, if the optimal I/O size is wrong, the read ahead size also is
wrong.  Can we just drop the setting?
Coly Li Sept. 21, 2020, 3:09 p.m. UTC | #3
On 2020/9/21 22:00, Christoph Hellwig wrote:
> On Mon, Sep 21, 2020 at 05:54:59PM +0800, Coly Li wrote:
>> I am not sure whether virtual bcache device's optimal request size can
>> be simply set like this.
>>
>> Most of time inherit backing device's optimal request size is fine, but
>> there are two exceptions,
>> - Read request hits on cache device
>> - User sets sequential_cuttoff as 0, all writing may go into cache
>> device firstly.
>> For the above two conditions, all I/Os goes into cache device, using
>> optimal request size of backing device might be improper.
>>
>> Just a guess, is it OK to set the optimal request size of the virtual
>> bcache device as the least common multiple of cache device's and backing
>> device's optimal request sizes ?
> 
> Well, if the optimal I/O size is wrong, the read ahead size also is
> wrong.  Can we just drop the setting?
> 

I feel this is something should be fixed. Indeed I overlooked it until
you point out the issue now.

The optimal request size and read ahead pages hint are necessary, but
current initialization is simple. A better way might be dynamically
setting them depends on the cache mode and some special configuration.

By your inspiration, I want to ACK your original patch although it
doesn't work fine for all condition. Then we may know these two settings
(ra_pages and queue_io_opt) should be improved for more situations. At
lease for most part of the situations they provide proper hints.

How do you think of the above idea ?

Coly Li
Christoph Hellwig Sept. 21, 2020, 6:18 p.m. UTC | #4
On Mon, Sep 21, 2020 at 11:09:48PM +0800, Coly Li wrote:
> I feel this is something should be fixed. Indeed I overlooked it until
> you point out the issue now.
> 
> The optimal request size and read ahead pages hint are necessary, but
> current initialization is simple. A better way might be dynamically
> setting them depends on the cache mode and some special configuration.
> 
> By your inspiration, I want to ACK your original patch although it
> doesn't work fine for all condition. Then we may know these two settings
> (ra_pages and queue_io_opt) should be improved for more situations. At
> lease for most part of the situations they provide proper hints.
> 
> How do you think of the above idea ?

Sounds like a plan.  I'd reall like to get this series in to get
some soaking before the end of the merge window, but we should still
have plenty of time for localized bcache updates.
Jan Kara Sept. 22, 2020, 8:44 a.m. UTC | #5
On Mon 21-09-20 10:07:24, Christoph Hellwig wrote:
> Inherit the optimal I/O size setting just like the readahead window,
> as any reason to do larger I/O does not apply to just readahead.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  drivers/md/bcache/super.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 1bbdc410ee3c51..48113005ed86ad 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1430,6 +1430,8 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
>  	dc->disk.disk->queue->backing_dev_info->ra_pages =
>  		max(dc->disk.disk->queue->backing_dev_info->ra_pages,
>  		    q->backing_dev_info->ra_pages);
> +	blk_queue_io_opt(dc->disk.disk->queue,
> +		max(queue_io_opt(dc->disk.disk->queue), queue_io_opt(q)));
>  
>  	atomic_set(&dc->io_errors, 0);
>  	dc->io_disable = false;
> -- 
> 2.28.0
>
Coly Li Sept. 22, 2020, 9:39 a.m. UTC | #6
On 2020/9/21 16:07, Christoph Hellwig wrote:
> Inherit the optimal I/O size setting just like the readahead window,
> as any reason to do larger I/O does not apply to just readahead.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Acked-by: Coly Li <colyli@suse.de>

Thanks.

Coly Li

> ---
>  drivers/md/bcache/super.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 1bbdc410ee3c51..48113005ed86ad 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1430,6 +1430,8 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
>  	dc->disk.disk->queue->backing_dev_info->ra_pages =
>  		max(dc->disk.disk->queue->backing_dev_info->ra_pages,
>  		    q->backing_dev_info->ra_pages);
> +	blk_queue_io_opt(dc->disk.disk->queue,
> +		max(queue_io_opt(dc->disk.disk->queue), queue_io_opt(q)));
>  
>  	atomic_set(&dc->io_errors, 0);
>  	dc->io_disable = false;
>
diff mbox series

Patch

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 1bbdc410ee3c51..48113005ed86ad 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1430,6 +1430,8 @@  static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
 	dc->disk.disk->queue->backing_dev_info->ra_pages =
 		max(dc->disk.disk->queue->backing_dev_info->ra_pages,
 		    q->backing_dev_info->ra_pages);
+	blk_queue_io_opt(dc->disk.disk->queue,
+		max(queue_io_opt(dc->disk.disk->queue), queue_io_opt(q)));
 
 	atomic_set(&dc->io_errors, 0);
 	dc->io_disable = false;