On 19.08.21 05:18, Qi Zheng wrote:
Hi,
This patch series aims to free user PTE page table pages when all PTE entries
are empty.
The beginning of this story is that some malloc libraries(e.g. jemalloc or
tcmalloc) usually allocate the amount of VAs by mmap() and do not unmap those VAs.
They will use madvise(MADV_DONTNEED) to free physical memory if they want.
But the page tables do not be freed by madvise(), so it can produce many
page tables when the process touches an enormous virtual address space.
The following figures are a memory usage snapshot of one process which actually
happened on our server:
VIRT: 55t
RES: 590g
VmPTE: 110g
As we can see, the PTE page tables size is 110g, while the RES is 590g. In
theory, the process only need 1.2g PTE page tables to map those physical
memory. The reason why PTE page tables occupy a lot of memory is that
madvise(MADV_DONTNEED) only empty the PTE and free physical memory but
doesn't free the PTE page table pages. So we can free those empty PTE page
tables to save memory. In the above cases, we can save memory about 108g(best
case). And the larger the difference between the size of VIRT and RES, the
more memory we save.
In this patch series, we add a pte_refcount field to the struct page of page
table to track how many users of PTE page table. Similar to the mechanism of
page refcount, the user of PTE page table should hold a refcount to it before
accessing. The PTE page table page will be freed when the last refcount is
dropped.
Testing:
The following code snippet can show the effect of optimization:
mmap 50G
while (1) {
for (; i < 1024 * 25; i++) {
touch 2M memory
madvise MADV_DONTNEED 2M
}
}
As we can see, the memory usage of VmPTE is reduced:
before after
VIRT 50.0 GB 50.0 GB
RES 3.1 MB 3.6 MB
VmPTE 102640 kB 248 kB
I also have tested the stability by LTP[1] for several weeks. I have not seen
any crash so far.
The performance of page fault can be affected because of the allocation/freeing
of PTE page table pages. The following is the test result by using a micro
benchmark[2]:
root@~# perf stat -e page-faults --repeat 5 ./multi-fault $threads:
threads before (pf/min) after (pf/min)
1 32,085,255 31,880,833 (-0.64%)
8 101,674,967 100,588,311 (-1.17%)
16 113,207,000 112,801,832 (-0.36%)
(The "pfn/min" means how many page faults in one minute.)
The performance of page fault is ~1% slower than before.
This series is based on next-20210812.
Comments and suggestions are welcome.
Thanks,
Qi.
Some high-level feedback after studying the code:
1. Try introducing the new dummy primitives ("API") first, and then
convert each subsystem individually; especially, maybe convert the whole
pagefault handling in a single patch, because it's far from trivial.
This will make this series much easier to digest.
Then, have a patch that adds actual logic to the dummy primitives via a
config option.
2. Minimize the API.
a) pte_alloc_get{,_map,_map_lock}() is really ugly. Maybe restrict it to
pte_alloc_get().
b) pmd_trans_unstable_or_pte_try_get() and friends are really ugly.
Handle it independently for now, even if it implies duplicate runtime
checks.
if (pmd_trans_unstable() || !pte_try_get()) ...
We can always optimize later, once we can come up with something cleaner.
3. Merge #6, and #7, after factoring out all changes to other subsystems
to use the API
4. Merge #8 into #6. There is a lot of unnecessary code churn back and
forth, and IMHO the whole approach might not make sense without RCU due
to the additional locking overhead.
Or at least, try to not modify the API you introduced in patch #6 or #7
in #8 again. Converting all call sites back and forth just makes review
quite hard.
I am preparing some some cleanups that will make get_locked_pte() and
similar a little nicer to handle. I'll send them out this or next week.
--
Thanks,
David / dhildenb
