linux-ext4 - Re: [RFC] performance regression with "ext4: Allow parallel DIO reads"

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Date:   Tue, 20 Aug 2019 12:08:05 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     Jan Kara <jack@...e.cz>, Joseph Qi <jiangqi903@...il.com>,
        Dave Chinner <david@...morbit.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Liu Bo <bo.liu@...ux.alibaba.com>
Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO
 reads"

On Tue, Aug 20, 2019 at 11:00:39AM +0800, Joseph Qi wrote:
> 
> I've tested parallel dio reads with dioread_nolock, it doesn't have
> significant performance improvement and still poor compared with reverting
> parallel dio reads. IMO, this is because with parallel dio reads, it take
> inode shared lock at the very beginning in ext4_direct_IO_read().

Why is that a problem?  It's a shared lock, so parallel threads should
be able to issue reads without getting serialized?

Are you using sufficiently fast storage devices that you're worried
about cache line bouncing of the shared lock?  Or do you have some
other concern, such as some other thread taking an exclusive lock?

      	       	       	    	  	 - Ted