Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table

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On 10.02.23 18:20, Chih-En Lin wrote:
On Fri, Feb 10, 2023 at 11:21:16AM -0500, Pasha Tatashin wrote:
Currently, copy-on-write is only used for the mapped memory; the child
process still needs to copy the entire page table from the parent
process during forking. The parent process might take a lot of time and
memory to copy the page table when the parent has a big page table
allocated. For example, the memory usage of a process after forking with
1 GB mapped memory is as follows:

For some reason, I was not able to reproduce performance improvements
with a simple fork() performance measurement program. The results that
I saw are the following:

Base:
Fork latency per gigabyte: 0.004416 seconds
Fork latency per gigabyte: 0.004382 seconds
Fork latency per gigabyte: 0.004442 seconds
COW kernel:
Fork latency per gigabyte: 0.004524 seconds
Fork latency per gigabyte: 0.004764 seconds
Fork latency per gigabyte: 0.004547 seconds

AMD EPYC 7B12 64-Core Processor
Base:
Fork latency per gigabyte: 0.003923 seconds
Fork latency per gigabyte: 0.003909 seconds
Fork latency per gigabyte: 0.003955 seconds
COW kernel:
Fork latency per gigabyte: 0.004221 seconds
Fork latency per gigabyte: 0.003882 seconds
Fork latency per gigabyte: 0.003854 seconds

Given, that page table for child is not copied, I was expecting the
performance to be better with COW kernel, and also not to depend on
the size of the parent.

Yes, the child won't duplicate the page table, but fork will still
traverse all the page table entries to do the accounting.
And, since this patch expends the COW to the PTE table level, it's not
the mapped page (page table entry) grained anymore, so we have to
guarantee that all the mapped page is available to do COW mapping in
the such page table.
This kind of checking also costs some time.
As a result, since the accounting and the checking, the COW PTE fork
still depends on the size of the parent so the improvement might not
be significant.

The current version of the series does not provide any performance
improvements for fork(). I would recommend removing claims from the
cover letter about better fork() performance, as this may be
misleading for those looking for a way to speed up forking. In my

 From v3 to v4, I changed the implementation of the COW fork() part to do
the accounting and checking. At the time, I also removed most of the
descriptions about the better fork() performance. Maybe it's not enough
and still has some misleading. I will fix this in the next version.
Thanks.

case, I was looking to speed up Redis OSS, which relies on fork() to
create consistent snapshots for driving replicates/backups. The O(N)
per-page operation causes fork() to be slow, so I was hoping that this
series, which does not duplicate the VA during fork(), would make the
operation much quicker.

Indeed, at first, I tried to avoid the O(N) per-page operation by
deferring the accounting and the swap stuff to the page fault. But,
as I mentioned, it's not suitable for the mainline.

Honestly, for improving the fork(), I have an idea to skip the per-page
operation without breaking the logic. However, this will introduce the
complicated mechanism and may has the overhead for other features. It
might not be worth it. It's hard to strike a balance between the
over-complicated mechanism with (probably) better performance and data
consistency with the page status. So, I would focus on the safety and
stable approach at first.

Yes, it is most probably possible, but complexity, robustness and maintainability have to be considered as well.

Thanks for implementing this approach (only deduplication without other optimizations) and evaluating it accordingly. It's certainly "cleaner", such that we only have to mess with unsharing and not with other accounting/pinning/mapcount thingies. But it also highlights how intrusive even this basic deduplication approach already is -- and that most benefits of the original approach requires even more complexity on top.

I am not quite sure if the benefit is worth the price (I am not to decide and I would like to hear other options).

My quick thoughts after skimming over the core parts of this series

(1) forgetting to break COW on a PTE in some pgtable walker feels quite
    likely (meaning that it might be fairly error-prone) and forgetting
    to break COW on a PTE table, accidentally modifying the shared
    table.
(2) break_cow_pte() can fail, which means that we can fail some
    operations (possibly silently halfway through) now. For example,
    looking at your change_pte_range() change, I suspect it's wrong.
(3) handle_cow_pte_fault() looks quite complicated and needs quite some
    double-checking: we temporarily clear the PMD, to reset it
    afterwards. I am not sure if that is correct. For example, what
    stops another page fault stumbling over that pmd_none() and
    allocating an empty page table? Maybe there are some locking details
    missing or they are very subtle such that we better document them. I
   recall that THP played quite some tricks to make such cases work ...


Actually, at the RFC v1 and v2, we proposed the version of skipping
those works, and we got a significant improvement. You can see the
number from RFC v2 cover letter [1]:
"In short, with 512 MB mapped memory, COW PTE decreases latency by 93%
for normal fork"

I suspect the 93% improvement (when the mapcount was not updated) was
only for VAs with 4K pages. With 2M mappings this series did not
provide any benefit is this correct?

Yes. In this case, the COW PTE performance is similar to the normal
fork().


The thing with THP is, that during fork(), we always allocate a backup PTE table, to be able to PTE-map the THP whenever we have to. Otherwise we'd have to eventually fail some operations we don't want to fail -- similar to the case where break_cow_pte() could fail now due to -ENOMEM although we really don't want to fail (e.g., change_pte_range() ).

I always considered that wasteful, because in many scenarios, we'll never ever split a THP and possibly waste memory.

Optimizing that for THP (e.g., don't always allocate backup THP, have some global allocation backup pool for splits + refill when close-to-empty) might provide similar fork() improvements, both in speed and memory consumption when it comes to anonymous memory.

--
Thanks,

David / dhildenb





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