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From: Richard Henderson <rth@twiddle.net>
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Date: Fri, 8 Apr 2016 11:48:30 -0700
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Subject: Re: [Qemu-devel] [PATCH v2 10/13] tb hash: hash phys_pc, pc,
 and flags with xxhash
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To: "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>
Cc: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Peter Maydell <peter.maydell@linaro.org>, Sergey Fedorov <serge.fdrv@gmail.com>

On 04/07/2016 10:32 AM, Emilio G. Cota wrote:
> For some workloads such as arm bootup, tb_phys_hash is performance-critical.
> The is due to the high frequency of accesses to the hash table, originated
> by (frequent) TLB flushes that wipe out the cpu-private tb_jmp_cache's.
> More info:
>   https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg05098.html
> 
> To dig further into this I modified an arm image booting debian jessie to
> immediately shut down after boot. Analysis revealed that quite a bit of time
> is unnecessarily spent in tb_phys_hash: the cause is poor hashing that
> results in very uneven loading of chains in the hash table's buckets;
> the longest observed chain had ~550 elements.
> 
> The appended addresses this with two changes:
> 
> 1) Use xxhash as the hash table's hash function. xxhash is a fast,
>    high-quality hashing function.
> 
> 2) Feed the hashing function with not just tb_phys, but also pc and flags.

Reviewed-by: Richard Henderson <rth@twiddle.net>


r~