In article <1174832185.251135.46220@l75g2000hse.googlegroups.c om>,
"sancho1980" wrote:

> Hi
> I was also reading about TLB's and multilevel page tables in osdi and
> now I'm starting to wonder if and how the TLB's could possibly be used
> in conjunction with multilevel page tables: the virtual address
> contains 3 indices: one into the top-level table, another one into the
> second-level table and yet another one into the actual page
> so if when i've cached a couple top-level page entries in my TLB's,
> then the top-level look-up would (hopefully) give me the base of a
> second level page table IN MEMORY, which is precisely the kind of
> thing I'm trying to avoid with the TLB, right? So, is there a way for
> an entry in the top-level entry to indicate that the second level page
> entry is in the TLB? do i Make sense, anyone got my point?
>
> thanks,
>
> Martin

The top-level table is pointers to the actual page table entries. The
TLB contains the actual page table entries.

If you have a hit in the TLB you needn't go to the page table at all.

Think of a 2-level page table as a 1-level page table that need not be
fully stored in memory.

1-level table:

entry 0 translation 0
entry 1 translation 1
entry 2 translation 2
entry 3 translation 3
:
:
:
entry N translation N

That's the whole page table.

Let's say for simplicity that the L1 table partitions the whole thing
into frames 4 entries long.

Then the top-level table is pointers to the 2nd-level table which will
consist of chunks like this:

entry 0 translation 0
entry 1 translation 1
entry 2 translation 2
entry 3 translation 3

Next chunk

entry 4 translation 4
entry 5 translation 5
entry 6 translation 6
entry 7 translation 2

Each line in the table is the same as the equivalent line in a 1-level
table. But because it is allocated in frames, it need not all exist at
once.

Hard to explain without pictures...

--
Philip Machanick

On Mar 26, 8:19 am, Philip Machanick wrote:
> In article <1174832185.251135.46...@l75g2000hse.googlegroups.c om>,
>
>
>
>
>
> "sancho1980" wrote:
> > Hi
> > I was also reading about TLB's and multilevel page tables in osdi and
> > now I'm starting to wonder if and how the TLB's could possibly be used
> > in conjunction with multilevel page tables: the virtual address
> > contains 3 indices: one into the top-level table, another one into the
> > second-level table and yet another one into the actual page
> > so if when i've cached a couple top-level page entries in my TLB's,
> > then the top-level look-up would (hopefully) give me the base of a
> > second level page table IN MEMORY, which is precisely the kind of
> > thing I'm trying to avoid with the TLB, right? So, is there a way for
> > an entry in the top-level entry to indicate that the second level page
> > entry is in the TLB? do i Make sense, anyone got my point?
>
> > thanks,
>
> > Martin
>
> The top-level table is pointers to the actual page table entries. The
> TLB contains the actual page table entries.
>
> If you have a hit in the TLB you needn't go to the page table at all.
>
> Think of a 2-level page table as a 1-level page table that need not be
> fully stored in memory.
>
> 1-level table:
>
> entry 0 translation 0
> entry 1 translation 1
> entry 2 translation 2
> entry 3 translation 3
> :
> :
> :
> entry N translation N
>
> That's the whole page table.
>
> Let's say for simplicity that the L1 table partitions the whole thing
> into frames 4 entries long.
>
> Then the top-level table is pointers to the 2nd-level table which will
> consist of chunks like this:
>
> entry 0 translation 0
> entry 1 translation 1
> entry 2 translation 2
> entry 3 translation 3
>
> Next chunk
>
> entry 4 translation 4
> entry 5 translation 5
> entry 6 translation 6
> entry 7 translation 2
>
> Each line in the table is the same as the equivalent line in a 1-level
> table. But because it is allocated in frames, it need not all exist at
> once.
>
> Hard to explain without pictures...
>
> --
> Philip Machanick- Hide quoted text -
>
> - Show quoted text -

Thank you for the explanation. I did actually understand the two-level
mechanism, what I didn't get was how the TLB could possibly be useful
when using multi-level page tables. But I got the answer later in the
chapter where it said that with multi-level page tables, the TLB
stores (first-level, second-level)-combinations and not just one
level!

Thanks anyway,

Martin