I've decided to do benchmarks to check how much SYSEMU saves in benchmark
which also access memory (memLoop.c) and how much could save the 0 context
switch idea (provided that segmentation has low cost).
First, about the benchmark on the Laurent Vivier page: I think that the "60 %"
number is meaningless - I guess it is that calculated with "real time", which
is not very meaningful IMHO - that is the time from when the process start to
when it ends, and counts even time spent by executing other processes. A more
meaningful difference is done with the sum of user+system time:
average time (user+system):
- without SYSEMU
64.910
- with SYSEMU
51.321
SYSEMU saves (64.910 - 51.321) / 64.910 * 100 % = 20,9 % of the time without
SYSEMU, in this benchmark.
I've re-benchmarked UML with SYSEMU using memLoop.c which tries to measure the
effects of accessing memory: it access one byte per page, thus causing the
CPU to reload in the TLB the page table entry (PTE) for that page. IMHO, this
benchmark shows that most of the gap vs the host is in the 2 remaining CS per
syscall: the 2 we save with SYSEMU account for about 25% of the getpid
execution, most of the gap is still there.
In the attached files NPAGES = 64 (see source), but I also posted results with
NPAGES = 512. Also, please, don't look at the "elapsed" time: it's
meaningless.
In fact getpidLoop measures only the cost of TLB flushes, while memLoop also
measures the cost of TLB misses after the TLB flush, which can be compared
against memLoopPure, which runs no syscall and thus never flushes the TLBs.
To see this, I must be sure that memLoopPure has no TLB fault, i.e. that the
PTEs for all pages fit in the TLB; this happen when NPAGES = 64, not when
NPAGES=512. In the two cases, we have working sets of 64 * PAGE_SIZE = 128k
and of 512 * PAGE_SIZE = 2 M.
On the host, memLoop and memLoopPure have similar user time, since there is
never a TLB flush. When NPAGES = 512, each page access causes a TLB miss, so
the user time is always similar, both on the host and the guest, and both
with and without syscalls.
But when NPAGES = 64, on the host the TLB is never flushed (except when
another process is executing): it is filled only once and then used.
On the guest, instead, with NPAGES = 64 the user time of memLoop is double
than the memLoopPure one. And since 0.40 s are for the getpid() calls,
touch_mem() uses 0.40 s in memLoopPure and 1.20 s in memLoop: 3 times the old
time.
--------
HOST:
host $ time ./getpidLoop 1000000
0.27user 0.21system 0:00.55elapsed 87%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (70major+11minor)pagefaults 0swaps
--------
With NPAGES = 64:
host $ time ./memLoop 1000000
1.11user 0.23system 0:01.46elapsed 91%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (79major+75minor)pagefaults 0swaps
----
host $ time ./memLoopPure 1000000
0.88user 0.00system 0:00.97elapsed 90%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (78major+75minor)pagefaults 0swaps
--------
With NPAGES = 512
host $ time ./memLoop 1000000
8.93user 0.24system 0:09.84elapsed 93%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (79major+523minor)pagefaults 0swaps
----
host $ time ./memLoopPure 1000000
8.71user 0.01system 0:09.43elapsed 92%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (78major+523minor)pagefaults 0swaps
------------
On the guest, with SYSEMU:
guest # /usr/bin/time
/mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/getpidLoop 1000000
0.42user 3.87system 0:16.09elapsed 26%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+76minor)pagefaults 0swaps
--------
With NPAGES = 64:
----
guest # /usr/bin/time /mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoop
1000000
1.60user 4.00system 0:18.02elapsed 31%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+146minor)pagefaults 0swaps
----
guest # /usr/bin/time
/mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoopPure 1000000
0.85user 0.05system 0:01.01elapsed 88%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+146minor)pagefaults 0swaps
--------
With NPAGES = 512:
guest # /usr/bin/time /mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoop
1000000
9.09user 4.18system 0:28.37elapsed 46%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+594minor)pagefaults 0swaps
----
guest # /usr/bin/time
/mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoopPure 1000000
8.76user 0.07system 0:11.57elapsed 76%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+594minor)pagefaults 0swaps
----------------
On the guest, without SYSEMU:
(we always about 25% increase for system time vs SYSEMU, except for
memLoopPure, but equal user time: we don't save the TLB misses)
# /usr/bin/time /mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/getpidLoop
1000000
0.42user 5.01system 0:21.08elapsed 25%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+76minor)pagefaults 0swaps
----
With NPAGES = 64:
guest # /usr/bin/time
/mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoopPure 1000000
(about the same, as expected)
0.86user 0.02system 0:00.94elapsed 92%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+146minor)pagefaults 0swaps
----
guest # /usr/bin/time /mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoop
1000000
(about 25% increase for system time, equal user time: we don't save the TLB
misses)
1.62user 5.00system 0:26.73elapsed 24%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+146minor)pagefaults 0swaps
--------
With NPAGES = 512
guest # /usr/bin/time
/mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoopPure 1000000
8.84user 0.02system 0:10.86elapsed 81%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+594minor)pagefaults 0swaps
----
guest # /usr/bin/time /mnt/host/home/paolo/Dati/Sorgenti/Varie/C-C++/memLoop
1000000
9.15user 5.06system 0:36.66elapsed 38%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+594minor)pagefaults 0swaps
--
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729
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