linux-kernel - Re: __GFP_REPEAT usage in fq_alloc

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Date:   Fri, 6 Jan 2017 08:50:31 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
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 Fri, Jan 6, 2017 at 8:48 AM, Eric Dumazet <edumazet@...gle.com> 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.

Forgot to mention tests include DDOS, so hitting a random hash bucket
for every packet.