Date: Thu, 26 Jul 2001 00:20:15 -0700 (PDT) From: Matt Dillon <dillon@earth.backplane.com> To: Rex Luo <rexluo@synology.com> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: qestion about vm page coloring Message-ID: <200107260720.f6Q7KFv54874@earth.backplane.com> References: <200107260408.MAA08931@synology.com>
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:Dear all,
:
: I study FreeBSD vm managememnt recently, however, I am a little confused
:with vm_page's page color. when you call vm_add_new_page() in vm_startup(),
:you will set each map entry's page color according to its physical addr.
:
: m->pc = (pa >> PAGE_SHIFT)&PQ_L2_MASK;
:
:However, I found that almost each map entry's page color is zero, that means
:PQ_L2_SIZE is 1, and disable page coloring option. Maybe I can do some
:modification to dump PQ_L2_SIZE's value, but I think my guess is right.
:Can someone please tell me the principle of page coloring, and why it's disabled
:now?
:
:Thanks,
:
:Rex Luo
I'm not sure what you mean by 'map entry'... vm_page_t's have color, and
vm_object's have a base color to randomly offset the color of the
vm_page_t's associated with the object, but vm_map_entry's do not have
a page color associated with them.
The page coloring works fine on my box, you may be looking at the wrong
thing. PQ_L2_SIZE is definitely not 1 unless you've specified some weird
kernel options in the kernel config.
-
Page coloring basically ensures that pages which are adjacent in
virtual memory also wind up being adjacent in the L1 and L2
cpu caches in order to get more consistent cpu cache behavior. Without
page coloring it is quite possible to have several adjacent pages in
virtual memory wind up utilizing the same cpu cache page, which can
effect performance with certain types of applications or certain cpu
cache topologies. On IA32 pentium architectures the effect would
probably not be all that noticeable, but getting consistent behavior
is still a good thing.
-Matt
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