Re: [PATCH v3 00/15] Free user PTE page table pages
From: Qi Zheng
Date: Thu Nov 11 2021 - 07:32:51 EST
On 11/11/21 8:20 PM, David Hildenbrand wrote:
On 11.11.21 13:00, Qi Zheng wrote:
On 11/11/21 7:19 PM, David Hildenbrand wrote:
On 11.11.21 12:08, Qi Zheng wrote:
On 11/11/21 5:22 PM, David Hildenbrand wrote:
On 11.11.21 04:58, Qi Zheng wrote:
On 11/11/21 1:37 AM, David Hildenbrand wrote:
It would still be a fairly coarse-grained locking, I am not sure if that
is a step into the right direction. If you want to modify *some* page
table in your process you have exclude each and every page table walker.
Or did I mis-interpret what you were saying?
That is one possible design, it favours fast walking and penalizes
mutation. We could also stick a lock in the PMD (instead of a
refcount) and still logically be using a lock instead of a refcount
scheme. Remember modify here is "want to change a table pointer into a
leaf pointer" so it isn't an every day activity..
It will be if we somewhat frequent when reclaim an empty PTE page table
as soon as it turns empty. This not only happens when zapping, but also
during writeback/swapping. So while writing back / swapping you might be
left with empty page tables to reclaim.
Of course, this is the current approach. Another approach that doesn't
require additional refcounts is scanning page tables for empty ones and
reclaiming them. This scanning can either be triggered manually from
user space or automatically from the kernel.
Whether it is introducing a special rwsem or scanning an empty page
table, there are two problems as follows:
#1. When to trigger the scanning or releasing?
For example when reclaiming memory, when scanning page tables in
khugepaged, or triggered by user space (note that this is the approach I
originally looked into). But it certainly requires more locking thought
to avoid stopping essentially any page table walker.
#2. Every time to release a 4K page table page, 512 page table
entries need to be scanned.
It would happen only when actually trigger reclaim of page tables
(again, someone has to trigger it), so it's barely an issue.
For example, khugepaged already scans the page tables either way.
For #1, if the scanning is triggered manually from user space, the
kernel is relatively passive, and the user does not fully know the best
timing to scan. If the scanning is triggered automatically from the
kernel, that is great. But the timing is not easy to confirm, is it
scanned and reclaimed every time zap or try_to_unmap?
For #2, refcount has advantages.
There is some advantage with this thinking because it harmonizes well
with the other stuff that wants to convert tables into leafs, but has
to deal with complicated locking.
On the other hand, refcounts are a degenerate kind of rwsem and only
help with freeing pages. It also puts more atomics in normal fast
paths since we are refcounting each PTE, not read locking the PMD.
Perhaps the ideal thing would be to stick a rwsem in the PMD. read
means a table cannot be come a leaf. I don't know if there is space
for another atomic in the PMD level, and we'd have to use a hitching
post/hashed waitq scheme too since there surely isn't room for a waitq
too..
I wouldn't be so quick to say one is better than the other, but at
least let's have thought about a locking solution before merging
refcounts :)
Yes, absolutely. I can see the beauty in the current approach, because
it just reclaims "automatically" once possible -- page table empty and
nobody is walking it. The downside is that it doesn't always make sense
to reclaim an empty page table immediately once it turns empty.
Also, it adds complexity for something that is only a problem in some
corner cases -- sparse memory mappings, especially relevant for some
memory allocators after freeing a lot of memory or running VMs with
memory ballooning after inflating the balloon. Some of these use cases
might be good with just triggering page table reclaim manually from user
space.
Yes, this is indeed a problem. Perhaps some flags can be introduced so
that the release of page table pages can be delayed in some cases.
Similar to the lazyfree mechanism in MADV_FREE?
The issue AFAIU is that once your refcount hits 0 (no more references,
no more entries), the longer you wait with reclaim, the longer others
have to wait for populating a fresh page table because the "page table
to be reclaimed" is still stuck around. You'd have to keep the refcount
increased for a while, and only drop it after a while. But when? And
how? IMHO it's not trivial, but maybe there is an easy way to achieve it.
For running VMs with memory ballooning after inflating the balloon, is
this a hot behavior? Even if it is, it is already facing the release and
reallocation of physical pages. The overhead after introducing
pte_refcount is that we need to release and re-allocate page table page.
But 2MB physical pages only corresponds to 4KiB of PTE page table page.
So maybe the overhead is not big.
The cases that come to my mind are
a) Swapping on shared memory with concurrent access
b) Reclaim on file-backed memory with concurrent access
c) Free page reporting as implemented by virtio-balloon
In all of these cases, you can have someone immediately re-access the
page table and re-populate it.
In the performance test shown on the cover, we repeatedly performed
touch and madvise(MADV_DONTNEED) actions, which simulated the case
you said above.
We did find a small amount of performance regression, but I think it is
acceptable, and no new perf hotspots have been added.
That test always accesses 2MiB and does it from a single thread. Things
might (IMHO will) look different when only accessing individual pages
and doing the access from one/multiple separate threads (that's what
No, it includes multi-threading:
while (1) {
char *c;
char *start = mmap_area[cpu];
char *end = mmap_area[cpu] + FAULT_LENGTH;
pthread_barrier_wait(&barrier);
//printf("fault into %p-%p\n",start, end);
for (c = start; c < end; c += PAGE_SIZE)
*c = 0;
pthread_barrier_wait(&barrier);
for (i = 0; cpu==0 && i < num; i++)
madvise(mmap_area[i], FAULT_LENGTH, MADV_DONTNEED);
pthread_barrier_wait(&barrier);
}
Thread on cpu0 will use madvise(MADV_DONTNEED) to release the physical
memory of threads on other cpu.
a),b) and c) essentially do, they don't do it in the pattern you
measured. what you measured matches rather a typical memory allocator).