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Re: [Qemu-devel] [PATCH v3 11/11] translate-all: add tb hash bucket info

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v3 11/11] translate-all: add tb hash bucket info to 'info jit' dump
Date: Fri, 22 Apr 2016 10:41:25 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 04/19/2016 04:07 PM, Emilio G. Cota wrote:
> +    ht_avg_len = qht_avg_bucket_chain_length(&tcg_ctx.tb_ctx.htable, 
> &ht_heads);
> +    cpu_fprintf(f, "TB hash avg chain   %0.5f buckets\n", ht_avg_len);
> +    cpu_fprintf(f, "TB hash size        %zu head buckets\n", ht_heads);

I think the accounting is questionable here.

Consider the following data:

TB count             230467/671088
TB invalidate count  25915
TB hash avg chain    1.03073 buckets
TB hash size         131072 head buckets

This means that we've got 230467 - 25915 = 204552 active TB's, installed into a
hash table with 131072 heads.  For a perfectly uniform distribution of TBs,
that would be an average chain length of 204552 / 131072 = 1.56.

In order to get the average down to 1.03, there need to be a substantial number
of heads with zero entries.

I think perhaps it might be more enlightening to separately account for empty
and non-empty heads.  E.g.

TB hash buckets used  xxxx/131072
TB hash avg chain     yyyy

where xxxx is the number of non-empty heads, and yyyy = |TBs| / xxxx.

I also wonder if it wouldn't be better to size the hash table as appropriate
for the maximum number of allowable TBs.


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