Re: [PATCH v2 0/9] Free user PTE page table pages

[David Hildenbrand <david@redhat.com>]

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