linux-kernel - Re: __GFP_REPEAT usage in fq_alloc

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Message-ID: <CANn89i+pRwa3KES1ane4ZfBpw4Y7Ne5OLZmkt=K8n5E6qF9xvA@mail.gmail.com>
Date:   Fri, 6 Jan 2017 09:08:43 -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 8:55 AM, Vlastimil Babka <vbabka@...e.cz> wrote:
> 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.
>

To be clear, the difference came from :

Using kmalloc() to allocate 48 x 524288 bytes

Or using vmalloc()

Are you telling me HugePages are not in play there ?