On Thu, Oct 1, 2020 at 10:36 PM Kirill A. Shutemov
[off-list ref] wrote:

On Thu, Oct 01, 2020 at 05:09:02PM -0700, Lokesh Gidra wrote:

quoted

On Thu, Oct 1, 2020 at 9:00 AM Kalesh Singh [off-list ref] wrote:

quoted

On Thu, Oct 1, 2020 at 8:27 AM Kirill A. Shutemov
[off-list ref] wrote:

quoted

On Wed, Sep 30, 2020 at 03:42:17PM -0700, Lokesh Gidra wrote:

quoted

On Wed, Sep 30, 2020 at 3:32 PM Kirill A. Shutemov
[off-list ref] wrote:

quoted

On Wed, Sep 30, 2020 at 10:21:17PM +0000, Kalesh Singh wrote:

quoted

mremap time can be optimized by moving entries at the PMD/PUD level if
the source and destination addresses are PMD/PUD-aligned and
PMD/PUD-sized. Enable moving at the PMD and PUD levels on arm64 and
x86. Other architectures where this type of move is supported and known to
be safe can also opt-in to these optimizations by enabling HAVE_MOVE_PMD
and HAVE_MOVE_PUD.

Observed Performance Improvements for remapping a PUD-aligned 1GB-sized
region on x86 and arm64:

    - HAVE_MOVE_PMD is already enabled on x86 : N/A
    - Enabling HAVE_MOVE_PUD on x86   : ~13x speed up

    - Enabling HAVE_MOVE_PMD on arm64 : ~ 8x speed up
    - Enabling HAVE_MOVE_PUD on arm64 : ~19x speed up

          Altogether, HAVE_MOVE_PMD and HAVE_MOVE_PUD
          give a total of ~150x speed up on arm64.

Is there a *real* workload that benefit from HAVE_MOVE_PUD?

We have a Java garbage collector under development which requires
moving physical pages of multi-gigabyte heap using mremap. During this
move, the application threads have to be paused for correctness. It is
critical to keep this pause as short as possible to avoid jitters
during user interaction. This is where HAVE_MOVE_PUD will greatly
help.

Any chance to quantify the effect of mremap() with and without
HAVE_MOVE_PUD?

I doubt it's a major contributor to the GC pause. I expect you need to
move tens of gigs to get sizable effect. And if your GC routinely moves
tens of gigs, maybe problem somewhere else?

I'm asking for numbers, because increase in complexity comes with cost.
If it doesn't provide an substantial benefit to a real workload
maintaining the code forever doesn't make sense.

mremap is indeed the biggest contributor to the GC pause. It has to
take place in what is typically known as a 'stop-the-world' pause,
wherein all application threads are paused. During this pause the GC
thread flips the GC roots (threads' stacks, globals etc.), and then
resumes threads along with concurrent compaction of the heap.This
GC-root flip differs depending on which compaction algorithm is being
used.

In our case it involves updating object references in threads' stacks
and remapping java heap to a different location. The threads' stacks
can be handled in parallel with the mremap. Therefore, the dominant
factor is indeed the cost of mremap. From patches 2 and 4, it is clear
that remapping 1GB without this optimization will take ~9ms on arm64.

Although this mremap has to happen only once every GC cycle, and the
typical size is also not going to be more than a GB or 2, pausing
application threads for ~9ms is guaranteed to cause jitters. OTOH,
with this optimization, mremap is reduced to ~60us, which is a totally
acceptable pause time.

Unfortunately, implementation of the new GC algorithm hasn't yet
reached the point where I can quantify the effect of this
optimization. But I can confirm that without this optimization the new
GC will not be approved.

IIUC, the 9ms -> 90us improvement attributed to combination HAVE_MOVE_PMD
and HAVE_MOVE_PUD, right? I expect HAVE_MOVE_PMD to be reasonable for some
workloads, but marginal benefit of HAVE_MOVE_PUD is in doubt. Do you see
it's useful for your workload?

Yes, 9ms -> 90us is when both are combined. The past experience has
been that even ~1ms long stop-the-world pause is prone to cause
jitters. HAVE_MOVE_PMD takes us only this far. So HAVE_MOVE_PUD is
required to bring the mremap cost to acceptable level.

Ideally, I was hoping that the functionality of HAVE_MOVE_PMD can be
extended to all levels of the hierarchical page table, and in the
process simplify the implementation. But unfortunately, that doesn't
seem to be possible from patch 3.

--
 Kirill A. Shutemov

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