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Re: Seeking consensus on BZ 16734

On 02/11/2015 03:02 PM, Rich Felker wrote:
> On Wed, Feb 11, 2015 at 02:34:31PM +0100, Florian Weimer wrote:
>> 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 <> 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.
> This seems like it would break horribly when the destination is
> anything but anonymous memory (presumably they checked that) and would
> perform atrociously bad (cost of locking vmas, possible TLB
> invalidation, etc.) especially when the size of the read is at most a
> few pages (which is the case for FILE buffers under normal usage). So
> it seems like, even if such a hack were possible, it was terribly
> misguided and would have pessimized performance for stdio.

I don't doubt that at all.  But perhaps that was the reason to
page-align the buffer.

You can see the details here:


(Curiously, this introduced a regression, fixed in

Florian Weimer / Red Hat Product Security

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