Re: [RFC PATCH] mm: readahead: add readahead_shift into backing device

[Mark Salyzyn <salyzyn@android.com>]

On 03/25/2019 05:16 AM, Fengguang Wu wrote:
> Martin,
>
> On Fri, Mar 22, 2019 at 11:46:11PM +0800, Martin Liu wrote:
>> As the discussion https://lore.kernel.org/patchwork/patch/334982/
>> We know an open file's ra_pages might run out of sync from
>> bdi.ra_pages since sequential, random or error read. Current design
>> is we have to ask users to reopen the file or use fdavise system
>> call to get it sync. However, we might have some cases to change
>> system wide file ra_pages to enhance system performance such as
>> enhance the boot time by increasing the ra_pages or decrease it to
>
> Do you have examples that some distro making use of larger ra_pages
> for boot time optimization?

Android (if you are willing to squint and look at android-common AOSP 
kernels as a Distro).

>
> Suppose N read streams with equal read speed. The thrash-free memory
> requirement would be (N * 2 * ra_pages).
>
> If N=1000 and ra_pages=1MB, it'd require 2GB memory. Which looks
> affordable in mainstream servers.
That is 50% of the memory on a high end Android device ...
>
> Sorry but it sounds like introducing an unnecessarily twisted new
> interface. I'm afraid it fixes the pain for 0.001% users while
> bringing more puzzle to the majority others.
 >2B Android devices on the planet is 0.001%?

I am not defending the proposed interface though, if there is something 
better that can be used, then looking into:
>
> Then let fadvise() and shrink_readahead_size_eio() adjust that
> per-file ra_pages_shift.
Sounds like this would require a lot from init to globally audit and 
reduce the read-ahead for all open files?

Sincerely -- Mark Salyzyn