This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: Seeking consensus on BZ 16734
- From: Florian Weimer <fweimer at redhat dot com>
- To: Rich Felker <dalias at libc dot org>, Paul Pluzhnikov <ppluzhnikov at google dot com>
- Cc: "H.J. Lu" <hjl dot tools at gmail dot com>, GLIBC Devel <libc-alpha at sourceware dot org>, Daniel Colascione <dancol at dancol dot org>, Andrew Pinski <pinskia at gmail dot com>
- Date: Wed, 11 Feb 2015 14:34:31 +0100
- Subject: Re: Seeking consensus on BZ 16734
- Authentication-results: sourceware.org; auth=none
- References: <CALoOobP_7jpdZUqSFmKCTFds6t8TTdnxfOfg2jCTr_TjvU+t2w at mail dot gmail dot com> <CAMe9rOrp6jCuPe4ZX-kdHdO_4_k-Dpf7ha-PxtCJmJLnL3K0-A at mail dot gmail dot com> <CALoOobMZFx7c+i0GCFRg1-1Z=2H3xDDH8+td-D=0k9muAFvPAA at mail dot gmail dot com> <20150202051410 dot GG23507 at brightrain dot aerifal dot cx>
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