On 12.01.24 11:18, Ryan Roberts wrote:
On 11/01/2024 13:18, David Hildenbrand wrote:
On 11.01.24 13:25, Ryan Roberts wrote:
On 10/01/2024 22:14, Barry Song wrote:
On Wed, Jan 10, 2024 at 7:59 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 10/01/2024 11:38, Barry Song wrote:
On Wed, Jan 10, 2024 at 7:21 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 10/01/2024 11:00, David Hildenbrand wrote:
On 10.01.24 11:55, Ryan Roberts wrote:
On 10/01/2024 10:42, David Hildenbrand wrote:
On 10.01.24 11:38, Ryan Roberts wrote:
On 10/01/2024 10:30, Barry Song wrote:
On Wed, Jan 10, 2024 at 6:23 PM Ryan Roberts <ryan.roberts@xxxxxxx>
wrote:
On 10/01/2024 09:09, Barry Song wrote:
On Wed, Jan 10, 2024 at 4:58 PM Ryan Roberts <ryan.roberts@xxxxxxx>
wrote:
On 10/01/2024 08:02, Barry Song wrote:
On Wed, Jan 10, 2024 at 12:16 PM John Hubbard
<jhubbard@xxxxxxxxxx> wrote:
On 1/9/24 19:51, Barry Song wrote:
On Wed, Jan 10, 2024 at 11:35 AM John Hubbard
<jhubbard@xxxxxxxxxx>
wrote:
...
Hi Ryan,
One thing that immediately came up during some recent testing
of mTHP
on arm64: the pid requirement is sometimes a little awkward. I'm
running
tests on a machine at a time for now, inside various
containers and
such, and it would be nice if there were an easy way to get some
numbers
for the mTHPs across the whole machine.
Just to confirm, you're expecting these "global" stats be truely
global
and not
per-container? (asking because you exploicitly mentioned being in a
container).
If you want per-container, then you can probably just create the
container
in a
cgroup?
I'm not sure if that changes anything about thpmaps here.
Probably
this is fine as-is. But I wanted to give some initial
reactions from
just some quick runs: the global state would be convenient.
Thanks for taking this for a spin! Appreciate the feedback.
+1. but this seems to be impossible by scanning pagemap?
so may we add this statistics information in kernel just like
/proc/meminfo or a separate /proc/mthp_info?
Yes. From my perspective, it looks like the global stats are
more useful
initially, and the more detailed per-pid or per-cgroup stats are
the
next level of investigation. So feels odd to start with the more
detailed stats.
probably because this can be done without the modification of the
kernel.
Yes indeed, as John said in an earlier thread, my previous
attempts to add
stats
directly in the kernel got pushback; DavidH was concerned that we
don't
really
know exectly how to account mTHPs yet
(whole/partial/aligned/unaligned/per-size/etc) so didn't want to
end up
adding
the wrong ABI and having to maintain it forever. There has also
been some
pushback regarding adding more values to multi-value files in
sysfs, so
David
was suggesting coming up with a whole new scheme at some point (I
know
/proc/meminfo isn't sysfs, but the equivalent files for NUMA nodes
and
cgroups
do live in sysfs).
Anyway, this script was my attempt to 1) provide a short term
solution
to the
"we need some stats" request and 2) provide a context in which to
explore
what
the right stats are - this script can evolve without the ABI problem.
The detailed per-pid or per-cgroup is still quite useful to my
case in
which
we set mTHP enabled/disabled and allowed sizes according to vma
types,
eg. libc_malloc, java heaps etc.
Different vma types can have different anon_name. So I can use the
detailed
info to find out if specific VMAs have gotten mTHP properly and
how many
they have gotten.
However, Ryan did clearly say, above, "In future we may wish to
introduce stats directly into the kernel (e.g. smaps or
similar)". And
earlier he ran into some pushback on trying to set up /proc or /sys
values because this is still such an early feature.
I wonder if we could put the global stats in debugfs for now?
That's
specifically supposed to be a "we promise *not* to keep this ABI
stable"
location.
Now that I think about it, I wonder if we can add a --global mode
to the
script
(or just infer global when neither --pid nor --cgroup are
provided). I
think I
should be able to determine all the physical memory ranges from
/proc/iomem,
then grab all the info we need from /proc/kpageflags. We should
then be
able to
process it all in much the same way as for --pid/--cgroup and
provide the
same
stats, but it will apply globally. What do you think?
Having now thought about this for a few mins (in the shower, if
anyone wants
the
complete picture :) ), this won't quite work. This approach doesn't
have the
virtual mapping information so the best it can do is tell us "how
many of
each
size of THP are allocated?" - it doesn't tell us anything about
whether they
are
fully or partially mapped or what their alignment is (all necessary
if we
want
to know if they are contpte-mapped). So I don't think this approach is
going to
be particularly useful.
And this is also the big problem if we want to gather stats inside the
kernel;
if we want something equivalant to /proc/meminfo's
AnonHugePages/ShmemPmdMapped/FilePmdMapped, we need to consider not
just the
allocation of the THP but also whether it is mapped. That's easy for
PMD-mappings, because there is only one entry to consider - when you
set it,
you
increment the number of PMD-mapped THPs, when you clear it, you
decrement.
But
for PTE-mappings it's harder; you know the size when you are mapping
so its
easy
to increment, but you can do a partial unmap, so you would need to
scan the
PTEs
to figure out if we are unmapping the first page of a previously
fully-PTE-mapped THP, which is expensive. We would need a cheap
mechanism to
determine "is this folio fully and contiguously mapped in at least one
process?".
as OPPO's approach I shared to you before is maintaining two mapcount
1. entire map
2. subpage's map
3. if 1 and 2 both exist, it is DoubleMapped.
This isn't a problem for us. and everytime if we do a partial unmap,
we have an explicit
cont_pte split which will decrease the entire map and increase the
subpage's mapcount.
but its downside is that we expose this info to mm-core.
OK, but I think we have a slightly more generic situation going on
with the
upstream; If I've understood correctly, you are using the PTE_CONT bit
in the
PTE to determne if its fully mapped? That works for your case where
you only
have 1 size of THP that you care about (contpte-size). But for the
upstream, we
have multi-size THP so we can't use the PTE_CONT bit to determine if
its fully
mapped because we can only use that bit if the THP is at least 64K and
aligned,
and only on arm64. We would need a SW bit for this purpose, and the mm
would
need to update that SW bit for every PTE one the full -> partial map
transition.
Oh no. Let's not make everything more complicated for the purpose of
some stats.
Indeed, I was intending to argue *against* doing it this way.
Fundamentally, if
we want to know what's fully mapped and what's not, then I don't see any
way
other than by scanning the page tables and we might as well do that in user
space with this script.
Although, I expect you will shortly make a proposal that is simple to
implement
and prove me wrong ;-)
Unlikely :) As you said, once you have multiple folio sizes, it stops really
making sense.
Assume you have a 128 kiB pageache folio, and half of that is mapped. You
can
set cont-pte bits on that half and all is fine. Or AMD can benefit from it's
optimizations without the cont-pte bit and everything is fine.
Yes, but for debug and optimization, its useful to know when THPs are
fully/partially mapped, when they are unaligned etc. Anyway, the script does
that for us, and I think we are tending towards agreement that there are
unlikely to be any cost benefits by moving it into the kernel.
frequent partial unmap can defeat all purpose for us to use large folios.
just imagine a large folio can soon be splitted after it is formed. we lose
the performance gain and might get regression instead.
nit: just because a THP gets partially unmapped in a process doesn't mean it
gets split into order-0 pages. If the folio still has all its pages mapped at
least once then no further action is taken. If the page being unmapped was the
last mapping of that page, then the THP is put on the deferred split queue, so
that it can be split in future if needed.
and this can be very frequent, for example, one userspace heap management
is releasing memory page by page.
In our real product deployment, we might not care about the second partial
unmapped, we do care about the first partial unmapped as we can use this
to know if split has ever happened on this large folios. an partial unmapped
subpage can be unlikely re-mapped back.
so i guess 1st unmap is probably enough, at least for my product. I mean we
care about if partial unmap has ever happened on a large folio more than how
they are exactly partially unmapped :-)
I'm not sure what you are suggesting here? A global boolean that tells you if
any folio in the system has ever been partially unmapped? That will almost
certainly always be true, even for a very well tuned system.
not a global boolean but a per-folio boolean. in case userspace maps a region
and has no userspace management, then we are fine as it is unlikely to have
partial unmap/map things; in case userspace maps a region, but manages it
by itself, such as heap things, we might result in lots of partial map/unmap,
which can lead to 3 problems:
1. potential memory footprint increase, for example, while userspace releases
some pages in a folio, we might still keep it as frequent splitting folio into
basepages and releasing the unmapped subpage might be too expensive.
2. if cont-pte is involved, frequent dropping cont-pte/tlb shootdown
might happen.
3. other maintenance overhead such as splitting large folios etc.
We'd like to know how serious partial map things are happening. so either
we will disable mTHP in this kind of VMAs, or optimize userspace to do
some alignment according to the size of large folios.
in android phones, we detect lots of apps, and also found some apps might
do things like
1. mprotect on some pages within a large folio
2. mlock on some pages within a large folio
3. madv_free on some pages within a large folio
4. madv_pageout on some pages within a large folio.
it would be good if we have a per-folio boolean to know how serious userspace
is breaking the large folios. for example, if more than 50% folios in a vma has
this problem, we can find it out and take some action.
The high level value of these stats seems clear - I agree we need to be able to
get these insights. I think the issues are more around the implementation
though. I'm struggling to understand exactly how we could implement a lot of
these things cheaply (either in the kernel or in user space).
Let me try to work though what I think you are suggesting:
- every THP is initially fully mapped
Not for pagecache folios.
- when an operation causes a partial unmap, mark the folio as having at least
one partial mapping
- on transition from "no partial mappings" to "at least one partial mapping"
increment a "anon-partial-<size>kB" (one for each supported folio size)
counter by the folio size
- on transition from "at least one partial mapping" to "fully unampped
everywhere" decrement the counter by the folio size
I think the issue with this is that a folio that is fully mapped in a process
that gets forked, then is partially unmapped in 1 process, will be accounted as
partially mapped even after the process that partially unmapped it exits, even
though that folio is now fully mapped in all processes that map it. Is that a
problem, perhaps not? I'm not sure.
What I can offer with my total mapcount I am working on (+ entire/pmd mapcount,
but let's put that aside):
Is "total mapcount" bound up as part of your "precise shared vs exclusive" work
or is it separate? If separate, do you have any ballpark feel for how likely it
is to land and if so, when?
You could have an expensive total mapcount via folio_mapcount() today.
The fast version is currently part of "precise shared vs exclusive", but
with most RMAP batching in place we might want to consider adding it
ahead of time, because the overhead of maintaining it will reduce
drastically in the cases we care about.
My current plan is:
(1) RMAP batching when remapping a PMD-mapped THP. Upstream.
(2) Fork batching. I have that prototype you already saw, will work
on this next.
(3) Zap batching. I also have a prototype now and will polish that
as well.
(4) Total mapcount
(5) Shared vs. Exclusive
(6) Subpage mapcounts / PageAnonExclusive fun
I'll try getting a per-folio PageAnonExclusive bit implemented ahead of
time. I think I know roughly what there is to do, but some corner cases
are ugly to handle and I avoided messing with them in the past by making
the PAE bit per-subpage. We'll see.
Now, no idea how long that all will take. I have decent prototypes at
this point for most stuff.
--
Cheers,
David / dhildenb
