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

[Chih-En Lin <shiyn.lin@gmail.com>]

On Sat, Feb 11, 2023 at 01:20:10AM +0800, 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

Sorry, it's "RFC v2 to v3".

> 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.

Thanks,
Chih-En Lin