linux-ext4 - Re: Discussion: is it time to remove dioread

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Date:   Fri, 27 Dec 2019 21:10:26 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     Liu Bo <bo.liu@...ux.alibaba.com>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
Subject: Re: Discussion: is it time to remove dioread_nolock?

Hi Ted,
After applying Ritesh's patches, from my test result, there is no
obvious performance difference between default mount options and
with dioread_lock (delalloc or nodelalloc).
I'm not sure if dioread_nolock was used for other purpose in the
scenario reported by Bo Liu. Maybe Xiaoguang could give some inputs.

Thanks,
Joseph


On 19/12/26 23:31, Theodore Y. Ts'o wrote:
> With inclusion of Ritesh's inode lock scalability patches[1], the
> traditional performance reasons for dioread_nolock --- namely,
> removing the need to take an exclusive lock for Direct I/O read
> operations --- has been removed.
> 
> [1] https://lore.kernel.org/r/20191212055557.11151-1-riteshh@linux.ibm.com
> 
> So... is it time to remove the code which supports dioread_nolock?
> Doing so would simplify the code base, and reduce the test matrix.
> This would also make it easier to restructure the write path when
> allocating blocks so that the extent tree is updated after writing out
> the data blocks, by clearing away the underbrush of dioread nolock
> first.
> 
> If we do this, we'd leave the dioread_nolock mount option for
> backwards compatibility, but it would be a no-op and not actually do
> anything.
> 
> Any objections before I look into ripping out dioread_nolock?
> 
> The one possible concern that I considered was for Alibaba, which was
> doing something interesting with dioread_nolock plus nodelalloc.  But
> looking at Liu Bo's explanation[2], I believe that their workload
> would be satisfied simply by using the standard ext4 mount options
> (that is, the default mode has the performance benefits when doing
> parallel DIO reads, and so the need for nodelalloc to mitigate the
> tail latency concerns which Alibaba was seeing in their workload would
> not be needed).  Could Liu or someone from Alibaba confirm, perhaps
> with some benchmarks using their workload?
> 
> [2] https://lore.kernel.org/linux-ext4/20181121013035.ab4xp7evjyschecy@US-160370MP2.local/
> 
>     	  	     	      	   	- Ted
>