Re: [PATCH 0/5] Speed up mremap on large regions

[Kirill A. Shutemov]

On Thu, Oct 01, 2020 at 05:09:02PM -0700, Lokesh Gidra wrote:
> On Thu, Oct 1, 2020 at 9:00 AM Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
> >
> > On Thu, Oct 1, 2020 at 8:27 AM Kirill A. Shutemov
> > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, Sep 30, 2020 at 03:42:17PM -0700, Lokesh Gidra wrote:
> > > > On Wed, Sep 30, 2020 at 3:32 PM Kirill A. Shutemov
> > > > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Wed, Sep 30, 2020 at 10:21:17PM +0000, Kalesh Singh wrote:
> > > > > > 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?

-- 
 Kirill A. Shutemov