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[lmi] KVM performance [Was: Compiling takes longer with gcc-4.9.2]

From: Greg Chicares
Subject: [lmi] KVM performance [Was: Compiling takes longer with gcc-4.9.2]
Date: Mon, 25 Jan 2016 00:47:45 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.3.0

On 2016-01-18 14:32, Vadim Zeitlin wrote:
> On Mon, 18 Jan 2016 05:04:23 +0000 Greg Chicares <address@hidden> wrote:
[...OLD vs "NEW" machines, msw-xp, gcc-4.9.1 ...]
> GC> Strangely, OLD beats NEW for building wx and wxPdfDoc, with --jobs=8.
> GC> Less strangely, NEW beats OLD for lmi makefile targets with --jobs=4;
> GC> but it's still a bit strange that the difference is so small.
>  Yes, both are indeed pretty strange but...
> GC> But what I really want to compare is cross compiling on x86_64-linux,
> ... it's this set of benchmarks that I'm most eager to see.

Time to build wx with g++-mingw-w64-i686 gcc-4.9.1 ...

Debian-8 running on bare metal, "OLD" machine, using a crucial m500 SSD:

  make --jobs=16 install
  2953.35s user 110.49s system 997% cpu 5:07.17 total
                                        ^^^^^^^ = 307s

Debian-8 guest, debian-7 host, qemu-kvm, "OLD" machine, WD "RE" HDD:
  make --jobs=8 install
  real 7m16.047s user 37m31.684s sys 2m18.176s
       ^^^^^^^^^ = 436s

Further data to make those numbers comparable:
  same machine, debian-7 on bare metal, cross-gcc-4.6:
  make --jobs=16 install  1673.20s user 139.40s system 1297% cpu 2:19.65 total 
  make --jobs=8  install  1089.86s user  99.95s system  708% cpu 2:47.96 total 
which suggests that 8 vs 16 vCPUs is a 140:168 speed ratio. Thus, if I
had given the VM 16 vCPUs, it might have taken
  436s * 140/168 = 363s
and thus (363 / 307) - 1 = 18% is the penalty for (VM, HDD) vs.
(bare metal, SSD). Sure, chroot would be faster, but it's not
like night and day.

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