Re: [RRC PATCH 2/2] vfs: Use per-cpu list for superblock's inode list

[Ingo Molnar]

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

> When many threads are trying to add or delete inode to or from
> a superblock's s_inodes list, spinlock contention on the list can
> become a performance bottleneck.
> 
> This patch changes the s_inodes field to become a per-cpu list with
> per-cpu spinlocks.
> 
> 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 (40 cores, 80 threads) were as
> follows:
> 
>   Kernel            Elapsed Time    System Time
>   ------            ------------    -----------
>   Vanilla 4.5-rc4      65.29s         82m14s
>   Patched 4.5-rc4      22.81s         23m03s
> 
> Before the patch, spinlock contention at the inode_sb_list_add()
> function at the startup phase and the inode_sb_list_del() function at
> the exit phase were about 79% and 93% of total CPU time respectively
> (as measured by perf). After the patch, the percpu_list_add()
> function consumed only about 0.04% of CPU time at startup phase. The
> percpu_list_del() function consumed about 0.4% of CPU time at exit
> phase. There were still some spinlock contention, but they happened
> elsewhere.

Pretty impressive IMHO!

Just for the record, here's your former 'batched list' number inserted into the 
above table:

   Kernel                       Elapsed Time    System Time
   ------                       ------------    -----------
   Vanilla      [v4.5-rc4]      65.29s          82m14s
   batched list [v4.4]          45.69s          49m44s
   percpu list  [v4.5-rc4]      22.81s          23m03s

i.e. the proper per CPU data structure and the resulting improvement in cache 
locality gave another doubling in performance.

Just out of curiosity, could you post the profile of the latest patches - is there 
any (bigger) SMP overhead left, or is the profile pretty flat now?

Thanks,

	Ingo