sendfile and page usage statistics

[Igor Sysoev]

On Wed, Oct 10, 2007 at 04:14:44PM +1000, Antony Mawer wrote:

> On 10/10/2007 4:06 PM, Igor Sysoev wrote:
> >On Wed, Oct 10, 2007 at 09:57:48AM +0400, Igor Sysoev wrote:
> >
> >>On Tue, Oct 09, 2007 at 04:50:01PM +0400, Igor Sysoev wrote:
> >>
> >>>As I understand if sendfile() with hardware TX chsum or TSO are used,
> >>>then CPU does not touch file pages at all. So pagedaemon never set
> >>>PG_REFERENCED to vnode object pages while scanning them.
> >>>Does VM set PG_REFERENCED by another way ? Otherwise, often used files
> >>>that accessed by sendfile() only have small chances to be cached by VM.
> >>I have looked kern_sendfile() in CURRENT and it seems to me that
> >>PG_REFERENCED is never set when sendfile() gets cached vm_page.
> >>I have looked the code path:
> >>
> >>vm_page_grab()
> >>    vm_page_lookup()
> >>    vm_page_wire()
> >>sf_buf_alloc() on i386 and amd64
> >>
> >>and have not seen PG_REFERENCED.
> >
> >As I understand the following patch against uipc_syscalls.c
> >should fix the bug to some extent:
> >
> >                         if (pg->valid && vm_page_is_valid(pg, pgoff, 
> >                         xfsize))
> >+                                vm_page_flag_set(pg, PG_REFERENCED);
> >                                 VM_OBJECT_UNLOCK(obj);
> >                         else if (m != NULL)
> 
> You are missing opening/closing braces on your if() statement with this 
> change...

Oh, thank you! You have saved me from coredump.
This is why in my coding style I always use braces for single statement 
inside if().


-- 
Igor Sysoev
http://sysoev.ru/en/