Re: [PATCH 0/4] mm/madvise: remove redundant mmap_lock operations from process_madvise()

[SeongJae Park <sj@kernel.org>]

Hi Vern,

On Tue, 11 Feb 2025 16:48:06 +0800 Vern Hao <haoxing990@gmail.com> wrote:

> 
> On 2025/2/6 14:15, SeongJae Park wrote:
> > process_madvise() calls do_madvise() for each address range.  Then, each
> > do_madvise() invocation holds and releases same mmap_lock.  Optimize the
> > redundant lock operations by splitting do_madvise() internal logics
> > including the mmap_lock operations, and calling the small logics
> > directly from process_madvise() in a sequence that removes the redundant
> > locking.  As a result of this change, process_madvise() becomes more
> > efficient and less racy in terms of its results and latency.
[...]
> >
> > Evaluation
> > ==========
> >
[...]
> > The measurement results are as below.  'sz_batches' column shows the
> > batch size of process_madvise() calls.  '0' batch size is for madvise()
> > calls case.
> Hi,  i just wonder why these patches can reduce latency time on call 
> madvise() DONT_NEED.

Thank you for asking this!

> >   'before' and 'after' columns are the measured time to apply
> > MADV_DONTNEED to the 256 MiB memory buffer in nanoseconds, on kernels
> > that built without and with the last patch of this series, respectively.
> > So lower value means better efficiency.  'after/before' column is the
> > ratio of 'after' to 'before'.
> >
> >      sz_batches  before       after        after/before
> >      0           146294215.2  121280536.2  0.829017989769427
> >      1           165851018.8  136305598.2  0.821855658085351
> >      2           129469321.2  103740383.6  0.801273866569094
> >      4           110369232.4  87835896.2   0.795836795182785
> >      8           102906232.4  77420920.2   0.752344327397609
> >      16          97551017.4   74959714.4   0.768415506038587
> >      32          94809848.2   71200848.4   0.750985786305689
> >      64          96087575.6   72593180     0.755489765942227
> >      128         96154163.8   68517055.4   0.712575022154163
> >      256         92901257.6   69054216.6   0.743307662177439
> >      512         93646170.8   67053296.2   0.716028168874151
> >      1024        92663219.2   70168196.8   0.75723892830177
[...]
> > Also note that this patch has somehow decreased latencies of madvise()
> > and single batch size process_madvise().  Seems this code path is small
> > enough to significantly be affected by compiler optimizations including
> > inlining of split-out functions.  Please focus on only the improvement
> > amount that changed by the batch size.

I believe the above paragraph may answer your question.  Please let me know if
not.


Thanks,
SJ

[...]