linux-kernel - Re: __GFP_REPEAT usage in fq_alloc

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Message-ID: <97be60da-72df-ad8f-db03-03f01c392823@suse.cz>
Date:   Fri, 6 Jan 2017 17:55:23 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: __GFP_REPEAT usage in fq_alloc_node

On 01/06/2017 05:48 PM, Eric Dumazet wrote:
> On Fri, Jan 6, 2017 at 8:31 AM, Vlastimil Babka <vbabka@...e.cz> wrote:
> 
>>
>> I wonder what's that cause of the penalty (when accessing the vmapped
>> area I suppose?) Is it higher risk of collisions cache misses within the
>> area, compared to consecutive physical adresses?
> 
> I believe tests were done with 48 fq qdisc, each having 2^16 slots.
> So I had 48 blocs,of 524288 bytes.
> 
> Trying a bit harder at setup time to get 128 consecutive pages got
> less TLB pressure.

Hmm that's rather surprising to me. TLB caches the page table lookups
and the PFN's of the physical pages it translates to shouldn't matter -
the page tables will look the same. With 128 consecutive pages could
manifest the reduced collision cache miss effect though.

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