Re: [Qemu-devel] [PATCH 2/8] migration: stop allocating and freeing memory frequently

[Peter Xu <peterx@redhat.com>]

On Thu, Mar 22, 2018 at 07:57:54PM +0800, Xiao Guangrong wrote:
> 
> 
> On 03/21/2018 05:06 PM, Peter Xu wrote:
> > On Tue, Mar 13, 2018 at 03:57:33PM +0800, guangrong.xiao@gmail.com wrote:
> > > From: Xiao Guangrong <xiaoguangrong@tencent.com>
> > > 
> > > Current code uses compress2()/uncompress() to compress/decompress
> > > memory, these two function manager memory allocation and release
> > > internally, that causes huge memory is allocated and freed very
> > > frequently
> > > 
> > > More worse, frequently returning memory to kernel will flush TLBs
> > > and trigger invalidation callbacks on mmu-notification which
> > > interacts with KVM MMU, that dramatically reduce the performance
> > > of VM
> > > 
> > > So, we maintain the memory by ourselves and reuse it for each
> > > compression and decompression
> > > 
> > > Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
> > > ---
> > >   migration/qemu-file.c |  34 ++++++++++--
> > >   migration/qemu-file.h |   6 ++-
> > >   migration/ram.c       | 142 +++++++++++++++++++++++++++++++++++++-------------
> > >   3 files changed, 140 insertions(+), 42 deletions(-)
> > > 
> > > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > > index 2ab2bf362d..1ff33a1ffb 100644
> > > --- a/migration/qemu-file.c
> > > +++ b/migration/qemu-file.c
> > > @@ -658,6 +658,30 @@ uint64_t qemu_get_be64(QEMUFile *f)
> > >       return v;
> > >   }
> > > +/* return the size after compression, or negative value on error */
> > > +static int qemu_compress_data(z_stream *stream, uint8_t *dest, size_t dest_len,
> > > +                              const uint8_t *source, size_t source_len)
> > > +{
> > > +    int err;
> > > +
> > > +    err = deflateReset(stream);
> > 
> > I'm not familiar with zlib, but I saw this in manual:
> > 
> >   https://www.zlib.net/manual.html
> > 
> >   This function is equivalent to deflateEnd followed by deflateInit,
> >   but does not free and reallocate the internal compression state. The
> >   stream will leave the compression level and any other attributes that
> >   may have been set unchanged.
> > 
> > I thought it was deflateInit() who is slow?  Can we avoid the reset as
> 
> deflateEnd() is worse as it frees memory to kernel which triggers
> TLB flush and mmu-notifier.
> 
> > long as we make sure to deflateInit() before doing anything else?
> 
> Actually, deflateReset() is cheap... :)
> 
> > 
> > Meanwhile, is there any performance number for this single patch?
> > Since I thought the old code is calling compress2() which contains
> > deflateInit() and deflateEnd() too, just like what current patch do?
> 
> No, after the patch, we just call deflateInit() / deflateEnd() one
> time (in _setup() handler and _cleanup handler).
> 
> Yes. This is the perf data from our production,
> after revert this patch:
> +  57.88%  kqemu  [kernel.kallsyms]        [k] queued_spin_lock_slowpath
> +  10.55%  kqemu  [kernel.kallsyms]        [k] __lock_acquire
> +   4.83%  kqemu  [kernel.kallsyms]        [k] flush_tlb_func_common
> 
> -   1.16%  kqemu  [kernel.kallsyms]        [k] lock_acquire                                       ▒
>    - lock_acquire                                                                                 ▒
>       - 15.68% _raw_spin_lock                                                                     ▒
>          + 29.42% __schedule                                                                      ▒
>          + 29.14% perf_event_context_sched_out                                                    ▒
>          + 23.60% tdp_page_fault                                                                  ▒
>          + 10.54% do_anonymous_page                                                               ▒
>          + 2.07% kvm_mmu_notifier_invalidate_range_start                                          ▒
>          + 1.83% zap_pte_range                                                                    ▒
>          + 1.44% kvm_mmu_notifier_invalidate_range_end
> 
> 
> apply our work:
> +  51.92%  kqemu  [kernel.kallsyms]        [k] queued_spin_lock_slowpath
> +  14.82%  kqemu  [kernel.kallsyms]        [k] __lock_acquire
> +   1.47%  kqemu  [kernel.kallsyms]        [k] mark_lock.clone.0
> +   1.46%  kqemu  [kernel.kallsyms]        [k] native_sched_clock
> +   1.31%  kqemu  [kernel.kallsyms]        [k] lock_acquire
> +   1.24%  kqemu  libc-2.12.so             [.] __memset_sse2
> 
> -  14.82%  kqemu  [kernel.kallsyms]        [k] __lock_acquire                                     ▒
>    - __lock_acquire                                                                               ▒
>       - 99.75% lock_acquire                                                                       ▒
>          - 18.38% _raw_spin_lock                                                                  ▒
>             + 39.62% tdp_page_fault                                                               ▒
>             + 31.32% __schedule                                                                   ▒
>             + 27.53% perf_event_context_sched_out                                                 ▒
>             + 0.58% hrtimer_interrupt
> 
> 
> You can see the TLB flush and mmu-lock contention have gone after this patch.

Yes.  Obviously I misunderstood the documentation for deflateReset().
It's not really a combined "End+Init", a quick glance in zlib code
shows that deflateInit() will do the mallocs, then call deflateReset()
at last.  So the buffers should be kept for reset(), as you explained.

> 
> > 
> > It would be nice too if we can split the patch into two (decode,
> > encode) if you want, but that's optional.
> 
> That's good to me, thank you, Peter.

Thanks for explaining.

-- 
Peter Xu