Re: [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table

Chih-En Lin
2022-05-21T18:50:16Z

On Sat, May 21, 2022 at 06:07:27PM +0200, David Hildenbrand wrote:
> On 19.05.22 20:31, Chih-En Lin wrote:
> > When creating the user process, it usually uses the Copy-On-Write (COW)
> > mechanism to save the memory usage and the cost of time for copying.
> > COW defers the work of copying private memory and shares it across the
> > processes as read-only. If either process wants to write in these
> > memories, it will page fault and copy the shared memory, so the process
> > will now get its private memory right here, which is called break COW.
>
> Yes. Lately we've been dealing with advanced COW+GUP pinnings (which
> resulted in PageAnonExclusive, which should hit upstream soon), and
> hearing about COW of page tables (and wondering how it will interact
> with the mapcount, refcount, PageAnonExclusive of anonymous pages) makes
> me feel a bit uneasy :)

I saw the series patch of this and knew how complicated handling COW of
the physical page was [1][2][3][4]. So the COW page table will tend to
restrict the sharing only to the page table. This means any modification
to the physical page will trigger the break COW of page table.

Presently implementation will only update the physical page information
to the RSS of the owner process of COW PTE. Generally owner is the
parent process. And the state of the page, like refcount and mapcount,
will not change under the COW page table.

But if any situations will lead to the COW page table needs to consider
the state of physical page, it might be fretful. ;-)

> >
> > Presently this kind of technology is only used as the mapping memory.
> > It still needs to copy the entire page table from the parent.
> > It might cost a lot of time and memory to copy each page table when the
> > parent already has a lot of page tables allocated. For example, here is
> > the state table for mapping the 1 GB memory of forking.
> >
> > mmap before fork mmap after fork
> > MemTotal: 32746776 kB 32746776 kB
> > MemFree: 31468152 kB 31463244 kB
> > AnonPages: 1073836 kB 1073628 kB
> > Mapped: 39520 kB 39992 kB
> > PageTables: 3356 kB 5432 kB
>
>
> I'm missing the most important point: why do we care and why should we
> care to make our COW/fork implementation even more complicated?
>
> Yes, we might save some page tables and we might reduce the fork() time,
> however, which specific workload really benefits from this and why do we
> really care about that workload? Without even hearing about an example
> user in this cover letter (unless I missed it), I naturally wonder about
> relevance in practice.
>
> I assume it really only matters if we fork() realtively large processes,
> like databases for snapshotting. However, fork() is already a pretty
> sever performance hit due to COW, and there are alternatives getting
> developed as a replacement for such use cases (e.g., uffd-wp).
>
> I'm also missing a performance evaluation: I'd expect some simple
> workloads that use fork() might be even slower after fork() with this
> change.
>

The paper mentioned a list of benchmarks of the time cost for On-Demand
fork. For example, on Redis, the meantime of fork when taking the
snapshot. Default fork() got 7.40 ms; On-demand Fork (COW PTE table) got
0.12 ms. But there are some other cases, like the Response latency
distribution of Apache HTTP Server, are not have significant benefits
from their On-demand fork.

For the COW page table from this patch, I also take the perf to analyze
the cost time. But it looks like not different from the default fork.

Here is the report, the mmap-sfork is COW page table version:

Performance counter stats for './mmap-fork' (100 runs):

373.92 msec task-clock # 0.992 CPUs utilized ( +- 0.09% )
1 context-switches # 2.656 /sec ( +- 6.03% )
0 cpu-migrations # 0.000 /sec
881 page-faults # 2.340 K/sec ( +- 0.02% )
1,860,460,792 cycles # 4.941 GHz ( +- 0.08% )
1,451,024,912 instructions # 0.78 insn per cycle ( +- 0.00% )
310,129,843 branches # 823.559 M/sec ( +- 0.01% )
1,552,469 branch-misses # 0.50% of all branches ( +- 0.38% )

0.377007 +- 0.000480 seconds time elapsed ( +- 0.13% )

Performance counter stats for './mmap-sfork' (100 runs):

373.04 msec task-clock # 0.992 CPUs utilized ( +- 0.10% )
1 context-switches # 2.660 /sec ( +- 6.58% )
0 cpu-migrations # 0.000 /sec
877 page-faults # 2.333 K/sec ( +- 0.08% )
1,851,843,683 cycles # 4.926 GHz ( +- 0.08% )
1,451,763,414 instructions # 0.78 insn per cycle ( +- 0.00% )
310,270,268 branches # 825.352 M/sec ( +- 0.01% )
1,649,486 branch-misses # 0.53% of all branches ( +- 0.49% )

0.376095 +- 0.000478 seconds time elapsed ( +- 0.13% )

So, the COW of the page table may reduce the time of forking. But it
builds on the transfer of the copy work to other modified operations
to the physical page.

> (I don't have time to read the paper, I'd expect an independent summary
> in the cover letter)

Sure, I will add more performance evaluations and descriptions in the
next version.

> I have tons of questions regarding rmap, accounting, GUP, page table
> walkers, OOM situations in page walkers, but at this point I am not
> (yet) convinced that the added complexity is really worth it. So I'd
> appreciate some additional information.

It seems like I have a lot of work to do. ;-)

>
> [...]
>
> > TODO list:
> > - Handle the swap
>
> Scary if that's not easy to handle :/

;-)

> --
> Thanks,
>
> David / dhildenb
>

Thanks!

[1] https://lore.kernel.org/all/20220131162940.210846-1-david@redhat.com/T/
[2] https://lore.kernel.org/linux-mm/20220315104741.63071-2-david@redhat.com/T/
[3] https://lore.kernel.org/linux-mm/51afa7a7-15c5-8769-78db-ed2d134792f4@redhat.com/T/
[4] https://lore.kernel.org/all/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@redhat.com/