[lkml]   [2007]   [Dec]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: RFC: remove __read_mostly
On Thu, Dec 13, 2007 at 11:48:27PM +0100, Eric Dumazet wrote:
> Adrian Bunk a écrit :
>> I tried the following patch with a full x86 .config [1]:
>> --- a/include/asm-x86/cache.h
>> +++ b/include/asm-x86/cache.h
>> -#define __read_mostly __attribute__((__section__(".data.read_mostly")))
>> +/* #define __read_mostly __attribute__((__section__(".data.read_mostly"))) */
>> The result [2,3] was:
>> -rwxrwxr-x 1 bunk bunk 46607243 2007-12-13 19:50 vmlinux.old
>> -rwxrwxr-x 1 bunk bunk 46598691 2007-12-13 21:55 vmlinux
>> It's not a surprise that the kernel can become bigger when __read_mostly
>> gets used, especially in cases where __read_mostly prevents gcc
>> optimizations.
>> My question is:
>> Is there anywhere in the kernel a case where __read_mostly brings a
>> measurable improvement or can it be removed?
> Yes, there are many cases where read_mostly brings huge improvements.
> I did test your idea on a 4 CPUS server, and system time was roughly
> doubled, from 11% to 20%

Thanks, that answered my question.

> Of course, you noticed that puting a __read_mostly attribute force the
> linker to reserve space for the variable. So a null variable previously in
> bss section (no space in vmlinux file) is now in .data.read_mostly. Not a
> big deal.
> If you want, you could play some .lds games to create sort of a
> "bss.read_mostly" section to save 10000 bytes in vmlinux.

No, what I was thinking of kernel code like

static int fooxxxvar = 0;
static int __read_mostly fooyyyvar = 0;

void mytest()
if (fooxxxvar)

if (fooyyyvar)

If you run "strings file.c | grep foo" on the file it will only return
"fooyyy" but not "fooxxx".

It's not a big deal, but it's an area where __read_mostly affects gcc
and not ld.



"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2007-12-14 00:03    [W:0.057 / U:3.904 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site