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Date:   Wed, 13 Jan 2021 11:44:30 -0500 (EST)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Zhongwei Cai <sunrise_l@sjtu.edu.cn>
cc:     Mingkai Dong <mingkaidong@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Eric Sandeen <esandeen@redhat.com>,
        Dave Chinner <dchinner@redhat.com>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Wang Jianchao <jianchao.wan9@gmail.com>,
        "Tadakamadla, Rajesh" <rajesh.tadakamadla@hpe.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-nvdimm@lists.01.org
Subject: Re: Expense of read_iter
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On Tue, 12 Jan 2021, Zhongwei Cai wrote:

> 
> I'm working with Mingkai on optimizations for Ext4-dax.

What specific patch are you working on? Please, post it somewhere.

> We think that optmizing the read-iter method cannot achieve the
> same performance as the read method for Ext4-dax. 
> We tried Mikulas's benchmark on Ext4-dax. The overall time and perf
> results are listed below:
> 
> Overall time of 2^26 4KB read.
> 
> Method       Time
> read         26.782s
> read-iter    36.477s

What happens if you use this trick ( https://lkml.org/lkml/2021/1/11/1612 )
- detect in the "read_iter" method that there is just one segment and 
treat it like a "read" method. I think that it should improve performance 
for your case.

Mikulas