linux-kernel - Re: [PATCH RFC] mm: Let readahead submit larger batches of pages in case of ra->ra

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Message-ID: <20200911094709.GB14158@infradead.org>
Date:   Fri, 11 Sep 2020 10:47:09 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Bean Huo <huobean@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, beanhuo@...ron.com,
        Richard Weinberger <richard@....at>
Subject: Re: [PATCH RFC] mm: Let readahead submit larger batches of pages in
 case of ra->ra_pages == 0

On Fri, Sep 11, 2020 at 10:15:24AM +0200, Bean Huo wrote:
> > What is special about ->ra_pages==0?  Wouldn't this optimization
> > still
> > be valid if ->ra_pages==2?
> > 
> > Doesn't this defeat the purpose of having ->ra_pages==0?
> 
> 
> Hi Andrew
> Sorry, I am still not quite understanding your above three questions. 
> 
> Based on my shallow understanding, ra_pages is associated with
> read_ahead_kb. Seems ra_pages controls the maximum read-ahead window
> size, but it doesn't work when the requested size exceeds ra_pages. 
> 
> If I set the read_ahead_kb to 0, also, as Christoph mentioned, MTD
> forcibly sets ra_pages to 0.  I think the intention is that only wants
> to disable read-ahead, however, doesn't want
> generic_file_buffered_read() to split the request and read data with
> 4KB chunk size separately.

They way I understood Richard this is intentional.