linux-kernel - Re: [PATCH v2 3/3] vfs: Enable list batching for the superblock's inode list

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Date:	Sat, 30 Jan 2016 09:35:57 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v2 3/3] vfs: Enable list batching for the superblock's
 inode list


* Waiman Long <Waiman.Long@....com> wrote:

> The inode_sb_list_add() and inode_sb_list_del() functions in the vfs
> layer just perform list addition and deletion under lock. So they can
> use the new list batching facility to speed up the list operations
> when many CPUs are trying to do it simultaneously.
> 
> In particular, the inode_sb_list_del() function can be a performance
> bottleneck when large applications with many threads and associated
> inodes exit. With an exit microbenchmark that creates a large number
> of threads, attachs many inodes to them and then exits. The runtimes
> of that microbenchmark with 1000 threads before and after the patch
> on a 4-socket Intel E7-4820 v3 system (48 cores, 96 threads) were
> as follows:
> 
>   Kernel        Elapsed Time    System Time
>   ------        ------------    -----------
>   Vanilla 4.4      65.29s         82m14s
>   Patched 4.4      45.69s         49m44s
> 
> The elapsed time and the reported system time were reduced by 30%
> and 40% respectively.

That's pretty impressive!

I'm wondering, why are inode_sb_list_add()/del() even called for a presumably 
reasonably well cached benchmark running on a system with enough RAM? Are these 
perhaps thousands of temporary files, already deleted, and released when all the 
file descriptors are closed as part of sys_exit()?

If that's the case then I suspect an even bigger win would be not just to batch 
the (sb-)global list fiddling, but to potentially turn the sb list into a 
percpu_alloc() managed set of per CPU lists? It's a bigger change, but it could 
speed up a lot of other temporary file intensive usecases as well, not just 
batched delete.

Thanks,

	Ingo