Re: [PATCH] btrfs: lower metadata writeback threshold on low dirtythreshold

[Fengguang Wu]

On Thu, May 03, 2012 at 11:43:11AM +0800, Fengguang Wu wrote:
> This helps write performance when setting the dirty threshold to tiny numbers.
> 
>      3.4.0-rc2         3.4.0-rc2-btrfs4+
>   ------------  ------------------------
>          96.92        -0.4%        96.54  bay/thresh=1000M/btrfs-100dd-1-3.4.0-rc2
>          98.47        +0.0%        98.50  bay/thresh=1000M/btrfs-10dd-1-3.4.0-rc2
>          99.38        -0.3%        99.06  bay/thresh=1000M/btrfs-1dd-1-3.4.0-rc2
>          98.04        -0.0%        98.02  bay/thresh=100M/btrfs-100dd-1-3.4.0-rc2
>          98.68        +0.3%        98.98  bay/thresh=100M/btrfs-10dd-1-3.4.0-rc2
>          99.34        -0.0%        99.31  bay/thresh=100M/btrfs-1dd-1-3.4.0-rc2
>   ==>    88.98        +9.6%        97.53  bay/thresh=10M/btrfs-10dd-1-3.4.0-rc2
>   ==>    86.99       +13.1%        98.39  bay/thresh=10M/btrfs-1dd-1-3.4.0-rc2
>   ==>     2.75     +2442.4%        69.88  bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2
>   ==>     3.31     +2634.1%        90.54  bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2

Well, further tests show that it behaves very unstable:

       3.4.0-rc2-btrfs4+         3.4.0-rc2-btrfs5+  
------------------------  ------------------------  
                   69.88       +16.4%        81.31  bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2-btrfs4+
                   71.09        +1.4%        72.05  bay/thresh=1M/btrfs-10dd-2-3.4.0-rc2-btrfs4+
                   72.60        -1.7%        71.38  bay/thresh=1M/btrfs-10dd-3-3.4.0-rc2-btrfs4+
                   90.54        -0.9%        89.74  bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2-btrfs4+
                   89.17       -90.2%     ==> 8.71  bay/thresh=1M/btrfs-1dd-2-3.4.0-rc2-btrfs4+
               ==> 14.96      +495.3%        89.06  bay/thresh=1M/btrfs-1dd-3-3.4.0-rc2-btrfs4+
                  408.23        +1.0%       412.26  TOTAL write_bw

Where the -btrfs5 kernel has one more patch to remove the write plug.

Thanks,
Fengguang
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