Subject: Re: [PATCH net-next 4/9] net: sctp: sctp_outq: consolidate chars into bitfield

From: Vlad Yasevich <[email-masked]>
Date: Wed, 17 Apr 2013 11:44:11 -0400

On 04/17/2013 05:26 AM, Daniel Borkmann wrote:
> On 04/17/2013 10:43 AM, David Laight wrote:
>>>       __u32 outstanding_bytes;
>>>
>>> -    /* Are we doing fast-rtx on this queue */
>>> -    char fast_rtx;
>>> -
>>> -    /* Corked? */
>>> -    char cork;
>>> -
>>> -    /* Is this structure empty?  */
>>> -    char empty;
>>> +    __u8 fast_rtx:1,    /* Are we doing fast-rtx on this queue */
>>> +         cork:1,        /* Corked? */
>>> +         empty:1;        /* Is this structure empty?  */
>>>   };
>>
>> Use of bitfields just makes the code slower.
>> The only real excuse for using them is to reduce the size
>> of a structure that is allocated a lot.
>
> sctp_outq is _embedded_ into an sctp_association structure, which
> has [size: 2280, cachelines: 36, members: 76]! A next step would be
> to try to reorder its elements carefully and see if we can reduce
> the size by filling some holes.

I remember attempting this undertaking that led me down the rabbit hole 
so fast that my head started to spin.  It is a badly needed project, but 
be prepared to spend a lot of time on it.

Not sure if the space saving is worth the bad code generated for 
bit-fields, especially considering that empty and cork are used on 
almost every packet.

-vlad

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