sendfile and page usage statistics

[Antony Mawer]

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...

--Antony