sendfile and page usage statistics

[Igor Sysoev]

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)


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