IRC, freenode, #hurd, 2011-09-02:

<slpz> what's the usual throughput for I/O operations (like "dd
  if=/dev/zero of=/dev/null") in one of those Xen based Hurd machines
<braunr> good question
<braunr> slpz: but don't use /dev/zero and /dev/null, as they don't have
  anything to do with true I/O operations
<slpz> braunr: in fact, I want to test the performance of IPC's virtual
  copy operations
<braunr> ok
<slpz> braunr: sorry, the "I/O" was misleading
<braunr> use bs=4096 then i guess
<slpz> bs > 2k
<braunr> ?
<slpz> braunr: everything about 2k is copied by vm_map_copyin/copyout
<slpz> s/about/above/
<slpz> braunr: MiG's stubs check for that value and generate complex (with
  out_of_line memory) messages if datalen is above 2k, IIRC
<braunr> ok
<braunr> slpz: found it, thanks
<tschwinge> tschwinge@strauss:~ $ dd if=/dev/zero of=/dev/null bs=4k & p=$!
  && sleep 10 && kill -s INFO $p && sleep 1 && kill $p
<tschwinge> [1] 13469
<tschwinge> 17091+0 records in
<tschwinge> 17090+0 records out
<tschwinge> 70000640 bytes (70 MB) copied, 17.1436 s, 4.1 MB/s
<tschwinge> Note, however 10 s vs. 17 s!
<tschwinge> And this is slow compared to heal hardware:
<tschwinge> thomas@coulomb:~ $ dd if=/dev/zero of=/dev/null bs=4k & p=$! &&
  sleep 10 && kill -s INFO $p && sleep 1 && kill $p
<tschwinge> [1] 28290
<tschwinge> 93611+0 records in
<tschwinge> 93610+0 records out
<tschwinge> 383426560 bytes (383 MB) copied, 9.99 s, 38.4 MB/s
<braunr> tschwinge: is the first result on xen vm ?
<tschwinge> I think so.
<braunr> :/
<slpz> tschwinge: Thanks! Could you please try with a higher block size,
  something like 128k or 256k?
<tschwinge> strauss is on a machine that also hosts a buildd, I think.
<braunr> oh ok
<pinotree> yes, aside either rossini or mozart
<tschwinge> And I can confirm that with dd if=/dev/zero of=/dev/null bs=4k
  running, a parallel sleep 10 takes about 20 s (on strauss).


<braunr> slpz: i'll set up xen hosts soon and can try those tests while
  nothing else runs to have more accurate results
<tschwinge> tschwinge@strauss:~ $ dd if=/dev/zero of=/dev/null bs=256k &
  p=$! && sleep 10 && kill -s INFO $p && sleep 1 && kill $p
<tschwinge> [1] 13482
<tschwinge> 4566+0 records in
<tschwinge> 4565+0 records out
<tschwinge> 1196687360 bytes (1.2 GB) copied, 13.6751 s, 87.5 MB/s
<braunr> slpz: gains are logarithmic beyond the page size
<tschwinge> thomas@coulomb:~ $ dd if=/dev/zero of=/dev/null bs=256k & p=$!
  && sleep 10 && kill -s INFO $p && sleep 1 && kill $p
<tschwinge> [1] 28295
<tschwinge> 6335+0 records in
<tschwinge> 6334+0 records out
<tschwinge> 1660420096 bytes (1.7 GB) copied, 9.99 s, 166 MB/s
<tschwinge> This time a the sleep 10 decided to take 13.6 s.
<slpz> tschwinge: Thanks again. The results for the Xen machine are not bad
  though. I can't obtain a throughput over 50MB/s with KVM.
<tschwinge> slpz: Want more data (bs)?  Just tell.
<braunr> slpz: i easily get more than that
<braunr> slpz: what buffer size do you use ?
<slpz> tschwinge: no, I just wanted to see if Xen has an upper limit beyond
  KVM's. Thank you.
<slpz> braunr: I try with different sizes until I find the maximum
  throughput for a certain amount of requests (count)
<slpz> braunr: are you working with KVM?
<braunr> yes
<braunr> slpz: my processor is a model name      : Intel(R) Core(TM)2 Duo
  CPU     E7500  @ 2.93GHz
<braunr> Linux silvermoon 2.6.32-5-amd64 #1 SMP Tue Jun 14 09:42:28 UTC
  2011 x86_64 GNU/Linux
<braunr> (standard amd64 squeeze kernel)
<slpz> braunr: and KVM's version?
<braunr> squeeze (0.12.5)
<braunr> bbl
<gnu_srs> 212467712 bytes (212 MB) copied, 9.95 s, 21.4 MB/s on kvm for me!
<slpz> gnu_srs: which block size?
<gnu_srs> 4k, and  61.7 MB/s with 256k
<slpz> gnu_srs: could you try with 512k and 1M?
<gnu_srs> 512k: 56.0 MB/s, 1024k: 40.2 MB/s Looks like the peak is around a
  few 100k
<slpz> gnu_srs: thanks!
<slpz> I've just obtained 1.3GB/s with bs=512k on other (newer) machine
<braunr> on which hw/vm ?
<slpz> I knew this is a cpu-bound test, but I couldn't imagine faster
  processors could make this difference
<slpz> braunr: Intel(R) Core(TM) i5 CPU         650  @ 3.20GHz
<slpz> braunr: KVM
<braunr> ok
<braunr> how much time did you wait before reading the result ?
<slpz> that was 20x times better than the same test on my Intel(R)
  Core(TM)2 Duo CPU     T7500  @ 2.20GHz
<slpz> braunr: I've repeated the test with a fixed "count"
<gnu_srs> My box is: Intel(R) Core(TM)2 Quad CPU    Q6600  @ 2.40GHz: Max
  is 67 MB/s around 140k block size
<braunr> yes but how much time did dd run ?
<gnu_srs>  10 s plus/minus a few fractions of a second,
<braunr> try waiting 30s
<slpz> braunr: didn't check, let me try again
<braunr> my kvm peaks at 130 MiB/s with bs 512k / 1M
<gnu_srs> 2029690880 bytes (2.0 GB) copied, 30.02 s, 67.6 MB/s, bs=140k
<braunr> gnu_srs: i'm very surprised with slpz's result of 1.3 GiB/s
<slpz> braunr: over 60 s running, same performance
<braunr> nice
<braunr> i wonder what makes it so fast
<braunr> how much cache ?
<gnu_srs> Me too, I cannot get better values than around 67 MB/s
<braunr> gnu_srs: same questions
<slpz> braunr: 4096KB, same as my laptop
<braunr> slpz: l2 ? l3 ?
<gnu_srs> kvm: cache=writeback, CPU: 4096 KB
<braunr> gnu_srs: this has nothing to do with the qemu option, it's about
  the cpu
<slpz> braunr: no idea, it's the first time I touch this machine. I going
  to see if I find the model in processorfinder
<braunr> under my host linux system, i get a similar plot, that is,
  performance drops beyond bs=1M
<gnu_srs> braunr: OK, bu I gave you the cache size too, same as slpz.
<braunr> i wonder what dd actually does
<braunr> read() and writes i guess
<slpz> braunr: read/write repeatedly, nothing fancy 
<braunr> slpz: i don't think it's a good test for virtual copy
<braunr> io_read_request, vm_deallocate, io_write_request, right
<braunr> slpz: i really wonder what it is about i5 that improves speed so
<slpz> braunr: me too
<slpz> braunr: L2: 2x256KB, L3: 4MB
<slpz> and something calling "SmartCache"
<gnu_srs> slpz: where did you find these values?
<slpz> gnu_srs: and wikipedia
<gnu_srs> aha, cpuinfo just gives cache size.
<slpz> that "SmartCache" thing seems to be just L2 cache sharing between
  cores. Shouldn't make a different since we're using only one core, and I
  don't see KVM hooping between them.
<manuel> with bs=256k: 7004487680 bytes (7.0 GB) copied, 10 s, 700 MB/s
<manuel> (qemu/kvm, 3 * Intel(R) Xeon(R) E5504 2GHz, cache size 4096 KB)
<slpz> manuel: did you try with 512k/1M?
<manuel> bs=512k: 7730626560 bytes (7.7 GB) copied, 10 s, 773 MB/s
<manuel> bs=1M: 7896825856 bytes (7.9 GB) copied, 10 s, 790 MB/s
<slpz> manuel: those are pretty good numbers too
<braunr> xeon processor
<gnu_srs> lshw gave me: L1 Cache 256KiB, L2 cache 4MiB
<slpz> sincerely, I've never seen Hurd running this fast. Just checked
  "uname -a" to make sure I didn't take the wrong image :-)
<manuel> for bs=256k, 60s: 40582250496 bytes (41 GB) copied, 60 s, 676 MB/s
<braunr> slpz: i think you can assume processor differences alter raw
  copies too much to get any valuable results about virtual copy operations
<braunr> you need a specialized test program
<manuel> and bs=512k, 60s, 753 MB/s
<slpz> braunr: I'm using the mach_perf suite from OSFMach to do the
  "serious" testing. I just wanted a non-synthetic test to confirm the

open issue gnumach -- have a look at mach_perf.

<braunr> manuel: how much cache ? 2M ?
<braunr> slpz: ok
<braunr> manuel: hmno, more i guess
<manuel> braunr: /proc/cpuinfo says cache size      : 4096 KB
<braunr> ok
<braunr> manuel: performance should drop beyond bs=2M
<braunr> but that's not relevant anyway
<gnu_srs> Linux:  bs=1M, 10.8 GB/s
<slpz> I think this difference is too big to be only due to a bigger amount
  of CPU cycles...
<braunr> slpz: clearly
<slpz> gnu_srs: your host system has 64 or 32 bits?
<slpz> braunr: I'm going to investigate a bit
<slpz> but this accidental discovery just made my day. We're able to run
  Hurd at decent speeds on newer hardware!
<braunr> slpz: what result do you get with the same test on your host
  system ?
<manuel> interestingly, running it several times has made the performance
  drop quite much (i'm getting 400-500MB/s with 1M now, compared to nearly
  800 fifteen minutes ago)


<slpz> braunr: probably an almost infinite throughput, but I don't consider
  that a valid test, since in Linux, the write operation to "/dev/null"
  doesn't involve memory copying/moving
<braunr> manuel: i observed the same behaviour
<gnu_srs> slpz: Host system is 64 bit
<braunr> slpz: it doesn't on the hurd either
<braunr> slpz: (under 2k, that is)
<braunr> over*
<slpz> braunr: humm, you're right, as the null translator doesn't "touch"
  the memory, CoW rules apply
<braunr> slpz: the only thing which actually copies things around is dd
<braunr> probably by simply calling read()
<braunr> which gets its result from a VM copy operation, but copies the
  content to the caller provided buffer
<braunr> then vm_deallocate() the data from the storeio (zero) translator
<braunr> if storeio isn't too dumb, it doesn't even touch the transfered
  buffer (as anonymous vm_map()ped memory is already cleared)

open issue documentation

<braunr> so this is a good test for measuring (profiling?) our ipc overhead
<braunr> and possibly the vm mapping operations (which could partly explain
  why the results get worse over time)
<braunr> manuel: can you run vminfo | wc -l on your gnumach process ?
<slpz> braunr: Yes, unless some special situation apply, like the source
  address/offset being unaligned, or if the translator decides to return
  the result in a different buffer (which I assume is not the case for
<manuel> braunr: 35
<braunr> slpz: they can't be unaligned, the vm code asserts that
<braunr> manuel: ok, this is normal
<slpz> braunr: address/offset from read()
<braunr> slpz: the caller provided buffer you mean ?
<slpz> braunr: yes, and the offset of the memory_object, if it's a pager
  based translator
<braunr> slpz: highly unlikely, the compiler chooses appropriate alignments
  for such buffers
<slpz> braunr: in those cases, memcpy is used over vm_copy
<braunr> slpz: and the glibc memcpy() optimized versions can usually deal
  with that
<braunr> slpz: i don't get your point about memory objects
<braunr> slpz: requests on memory objects always have aligned values too
<slpz> braunr: sure, but can't deal with the user requesting non
  page-aligned sizes
<braunr> slpz: we're considering our dd tests, for which we made sure sizes
  were page aligned
<slpz> braunr: oh, I was talking in a general sense, not just in this dd
  tests, sorry
<slpz> by the way, dd on the host tops at 12 GB/s with bs=2M
<braunr> that's consistent with our other results
<braunr> slpz: you mean, even on your i5 processor with 1.3 GiB/s on your
  hurd kvm ?
<slpz> braunr: yes, on the GNU/Linux which is running as host
<braunr> slpz: well that's not consistent
<slpz> braunr: consistent with what?
<braunr> slpz: i get roughly the same result on my host, but ten times less
  on my hurd kvm
<braunr> slpz: what's your kernel/kvm versions ?
<slpz> 2.6.32-5-amd64 (debian's build) 0.12.5
<braunr> same here
<braunr> i'm a bit clueless
<braunr> why do i only get 130 MiB/s where you get 1.3 .. ? :)
<slpz> well, on my laptop, where Hurd on KVM tops on 50 MB/s, Linux gets a
  bit more than 10 GB/s
<braunr> see
<braunr> slpz: reduce bs to 256k and test again if you have time please
<slpz> braunr: on which system?
<braunr> slpz: the fast one
<braunr> (linux host)
<slpz> braunr: Hurd?
<slpz> ok
<slpz> 12 GB/s
<braunr> i get 13.3
<slpz> same for 128k, only at 64k starts dropping
<slpz> maybe, on linux we're being limited by memory speed, while on Hurd's
  this test is (much) more CPU-bound?
<braunr> slpz: maybe
<braunr> too bad processor stalls aren't easy to measure
<slpz> braunr: that's very true. It's funny when you read a paper which
  measures performance by cycles on an old RISC processor. That's almost
  impossible to do (with reliability) nowadays :-/
<slpz> I wonder which throughput can achieve Hurd running bare-metal on
  this machine...
<antrik> both the Xeon and the i5 use cores based on the Nehalem
<antrik> apparently Nehalem is where Intel first introduces nested page
<antrik> which pretty much explains the considerably lower overhead of VM
<cjuner> antrik, what are nested page tables? (sounds like the 4-level page
  tables we already have on amd64, or 2-level or 3-level on x86 pae)
<antrik> page tables were always 2-level on x86
<antrik> that's unrelated
<antrik> nested page tables means there is another layer of address
  translation, so the VMM can do it's own translation and doesn't care what
  the guest system does => no longer has to intercept all page table
<braunr> antrik: do you imply it only applies to virtualized systems ?
<antrik> braunr: yes
<slpz> antrik: Good guess. Looks like Intel's EPT are doing the trick by
  allowing the guest OS deal with its own page faults
<slpz> antrik: next monday, I'll try disabling EPT support in KVM on that
  machine (the fast one). That should confirm your theory empirically.
<slpz> this also means that there're too many page faults, as we should be
  doing virtual copies of memory that is not being accessed
<slpz> and looking at how the value of "page faults" in "vmstat" increases,
  shows that page faults are directly proportional to the number of pages
  we are asking from the translator
<slpz> I've also tried doing a long read() directly, to be sure that "dd"
  is not doing something weird, and it shows the same behaviour.
<braunr> slpz: dd does copy buffers
<braunr> slpz: i told you, it's not a good test case for pure virtual copy
<braunr> antrik: do you know if xen benefits from nested page tables ?
<antrik> no idea

open issue xen

<slpz> braunr: but my small program doesn't, and still provokes a lot of
  page faults
<braunr> slpz: are you certain it doesn't ?
<slpz> braunr: looking at google, it looks like recent Xen > 3.4 supports
<braunr> ok
<braunr> i'm ordering my new server right now, core i5 :)
<slpz> braunr: at least not explicitily. I need to look at MiG stubs again,
  I don't remember if they do something weird.
<antrik> braunr: sandybridge or nehalem? :-)
<braunr> antrik: no idea
<antrik> does it tell a model number?
<braunr> not yet
<braunr> but i don't have a choice for that, so i'll order it first, check
<antrik> hehe
<antrik> I'm not sure it makes all that much difference anyways for a
  server... unless you are running it at 100% load ;-)
<braunr> antrik: i'm planning on running xen guests suchs as new buildd
<antrik> hm... note though that some of the nehalem-generation i5s were
  dual-core, while all the new ones are quad
<braunr> it's a quad
<antrik> the newer generation has better performance per GHz and per
  Watt... but considering that we are rather I/O-limited in most cases, it
  probably won't make much difference
<antrik> not sure whether there are further virtualisation improvements
  that could be relevant...
<braunr> buildds spend much time running gcc, so even such improvements
  should help
<braunr> there, server ordered :)
<braunr> antrik: model name      : Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz

IRC, freenode, #hurd, 2011-09-06:

<slpz> youpi: what machines are being used for buildd? Do you know if they
  have EPT/RVI?
<youpi> we use PV Xen there
<slpz> I think Xen could also take advantage of those technologies. Not
  sure if only in HVM or with PV too.
<youpi> only in HVM
<youpi> in PV it does not make sense: the guest already provides the
  translated page table
<youpi> which is just faster than anything else

IRC, freenode, #hurd, 2011-09-09:

<antrik> oh BTW, for another data point: dd zero->null gets around 225 MB/s
  on my lowly 1 GHz Pentium3, with a blocksize of 32k
<antrik> (but only half of that with 256k blocksize, and even less with 1M)
<antrik> the system has been up for a while... don't know whether it's
  faster on a freshly booted one

IRC, freenode, #hurd, 2011-09-15:

<sudoman> so is the dd command pointed to by that article a measure of io
<antrik> sudoman: no, not really
<antrik> it's basically the baseline of what is possible -- but the actual
  slowness we experience is more due to very unoptimal disk access patterns
<antrik> though using KVM with writeback caching does actually help with
<antrik> also note that the title of this post really makes no
  sense... nested page tables should provide similar improvements for *any*
  guest system doing VM manipulation -- it's not Hurd-specific at all
<sudoman> ok, that makes sense. thanks :)

IRC, freenode, #hurd, 2011-09-16:

<slpz> antrik: I wrote that article (the one about How AMD/Intel fixed...)
<slpz> antrik: It's obviously a bit of an exaggeration, but it's true that
  nested pages supposes a great improvement in the performance of Hurd
  running on virtual machines
<slpz> antrik: and it's Hurd specific, as this system is more affected by
  the cost of page faults
<slpz> antrik: and as the impact of virtualization on the performance is
  much higher than (almost) any other OS.
<slpz> antrik: also, dd from /dev/zero to /dev/null it's a measure on how
  fast OOL IPC is.