Re: [PATCH v2 2/2] fat: improve fat_sync_bhs() write submission

[OGAWA Hirofumi]

Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx> writes:

> fat_sync_bhs() does many individual write_dirty_buffer() calls without
> plugging, making batching in the block layer more difficult. It also
> does not pass the REQ_SYNC hint for these synchronous writes.
>
> Use a plug around the writes so the block layer can more easily batch
> them. Pass the REQ_SYNC hint to write_dirty_buffer().
>
> Benchmarking was done under QEMU/KVM with a virtio-blk device using
> cache=directsync. Each result is the arithmetic mean of 10 samples.
>
> start = now_ns();
> for (unsigned i = 0; i < count; i++) {
>     fallocate(fds[i], FALLOC_FL_KEEP_SIZE, 0, bytes);
> }
> elapsed = now_ns() - start;
>
> Sync-mounted FAT32 using count=16 files:
> File size    Cluster size    Before    After    Improvement
> 256MiB       512B            13.11s    11.87s   1.10x
> 256MiB       4096B           1.657s    1.469s   1.13x
>
> Signed-off-by: Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx>

Acked-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>

Thanks.

> ---
>  fs/fat/misc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fat/misc.c b/fs/fat/misc.c
> index b6a9e2e122cd..e8e2f13254d5 100644
> --- a/fs/fat/misc.c
> +++ b/fs/fat/misc.c
> @@ -9,6 +9,7 @@
>  
>  #include "fat.h"
>  #include <linux/iversion.h>
> +#include <linux/blk_plug.h>
>  
>  /*
>   * fat_fs_error reports a file system problem that might indicate fa data
> @@ -354,9 +355,14 @@ EXPORT_SYMBOL_GPL(fat_update_time);
>  int fat_sync_bhs(struct buffer_head **bhs, int nr_bhs)
>  {
>  	int i, err = 0;
> +	struct blk_plug plug;
> +
> +	blk_start_plug(&plug);
>  
>  	for (i = 0; i < nr_bhs; i++)
> -		write_dirty_buffer(bhs[i], 0);
> +		write_dirty_buffer(bhs[i], REQ_SYNC);
> +
> +	blk_finish_plug(&plug);
>  
>  	for (i = 0; i < nr_bhs; i++) {
>  		wait_on_buffer(bhs[i]);

-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>