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

[Fengguang Wu <fengguang.wu@intel.com>]

On Mon, Mar 25, 2019 at 09:59:31AM -0700, Mark Salyzyn wrote:
>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).

OK. I wonder how exactly Android makes use of it. Since phones are not
using hard disks, so should benefit less from large ra_pages.  Would
you kindly point me to the code?

>> 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 ...

Yeah but I'm obviously not talking Android device here. Will a phone
serve 1000 concurrent read streams?

>> 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%?

Nope. Sorry I didn't know about the Android usage.
Actually nobody mentioned it in the past discussions.

>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?

It depends. In theory it should be possible to create a standalone
kernel module to dump the page cache and get the current snapshot of
all cached file pages. It'd be a one-shot action and don't require
continuous auditing.

[RFC] kernel facilities for cache prefetching
https://lwn.net/Articles/182128

This tool may also work. It's quick to get the list of opened files by
walking /proc/*/fd/, however not as easy to get the list of cached
file names.

https://github.com/tobert/pcstat

Perhaps we can do a simplified /proc/filecache that only dumps the
list of cached file names. Then let mincore() based tools take care
of the rest work.

Regards,
Fengguang