Re: [PATCH] mm: add config for readahead window

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Mar 16, 2018 at 11:25:08AM -0700, Wei Wang wrote:
> From: Wei Wang <wvw@google.com>
> 
> Change VM_MAX_READAHEAD value from the default 128KB to a configurable
> value. This will allow the readahead window to grow to a maximum size
> bigger than 128KB during boot, which could benefit to sequential read
> throughput and thus boot performance.

Increase of readahead window was proposed several times. And rejected.
IIRC, Linus was against it.

What is different this time?

> Signed-off-by: Wei Wang <wvw@google.com>
> ---
>  include/linux/mm.h | 2 +-
>  mm/Kconfig         | 8 ++++++++
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ad06d42adb1a..d7dc6125833e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2291,7 +2291,7 @@ int __must_check write_one_page(struct page *page);
>  void task_dirty_inc(struct task_struct *tsk);
>  
>  /* readahead.c */
> -#define VM_MAX_READAHEAD	128	/* kbytes */
> +#define VM_MAX_READAHEAD	CONFIG_VM_MAX_READAHEAD_KB
>  #define VM_MIN_READAHEAD	16	/* kbytes (includes current page) */
>  
>  int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
> diff --git a/mm/Kconfig b/mm/Kconfig
> index c782e8fb7235..da9ff543bdb9 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -760,3 +760,11 @@ config GUP_BENCHMARK
>  	  performance of get_user_pages_fast().
>  
>  	  See tools/testing/selftests/vm/gup_benchmark.c
> +
> +config VM_MAX_READAHEAD_KB
> +	int "Default max readahead window size in Kilobytes"
> +	default 128
> +	help
> +	  This sets the VM_MAX_READAHEAD value to allow the readahead window
> +	  to grow to a maximum size of configured. Increasing this value will
> +	  benefit sequential read throughput.
> -- 
> 2.16.2.804.g6dcf76e118-goog
> 

-- 
 Kirill A. Shutemov