Re: Performance degradation on 4.6

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Hi Robert,

Adding HCH + Sagi CC, as there have been significant changes made in
RDMA for v4.6 code.

On Tue, 2016-05-31 at 17:46 -0600, Robert LeBlanc wrote:
> I've been working with iSER trying to maximize performance. I have two
> FDR Infiniband cards (three ports total) and I've created multiple
> address on the target and configured 12 paths from the initiator. The
> iSER target is served up from a RAM disk. With the 4.4 kernel I'm able
> to pretty much saturate the links (4.1 million seq 4K read IOPs 15.7
> GB/s) using multipathd. With the same configuration on 4.6, I'm only
> able to get 3.3 million IOPs or 12.8 GB/s.
> 
> Running perf top on the target and initiator, I'm seeing the target is
> dominated by
> 
> 73.89% [kernel] [k] memcpy_rems
>  1.35% [kernel] [k] get_page_from_freelist
> ...
> 
> which is the same with 4.4. Perf top on the target has:
> 
> 13.01% [kernel] [k] copy_user_enhanced_fast_string
>  8.48% [kernel] [K] __blk_recalc_re_segments
>  5.35% [kernel] [k] native_queued_spin_lock_slowpath
> ...
> 
> I don't recall what the initiator had on 4.4.
> 
> Single path performance has increase by about 5% which is nice, but
> two path performance is about 50% that of the 4.4 kernel. Three paths
> and greater are around 80% the 4.4 performance.
> 
> Any ideas on what could have caused the performance issues on 4.6? Is
> there some additional information I can provide that would be useful?
> 
> Thanks,
> 
> ----------------
> Robert LeBlanc
> PGP Fingerprint 79A2 9CA4 6CC4 45DD A904  C70E E654 3BB2 FA62 B9F1
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