Seeking consensus on BZ 16734

[Florian Weimer]

On 02/02/2015 06:14 AM, Rich Felker wrote:
> On Sun, Feb 01, 2015 at 08:46:06PM -0800, Paul Pluzhnikov wrote:
>> On Sun, Feb 1, 2015 at 8:09 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>>>> Can we just do it?
>>>>
>>>
>>> Do we have any current performance data on this?
>>
>> I am not sure what performance data you want.
>>
>> The application CPU will go up (calloc has to zero out space), kernel
>> CPU will go down (kernel would not have to zero out the same space).
>>
>> It's clear that calloc()ing 8K is much cheaper than mmap()ing,
>> especially when there are 100s of threads.
> 
> The original idea seems to be some misguided idea that read/write
> should perform better with a page-aligned buffer.

Historically, some Linux VFS read implementations could transfer the
data by mapping full pages (/dev/zero was one of them).  I think they
have been gone for a long time because you need to copy lots and lots of
data (certainly more than 8K) before you lose against remapping and the
cache invalidation that comes with it.

-- 
Florian Weimer / Red Hat Product Security