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 (*bber)? <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>  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>  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>  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>  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. ``Interesting.'' <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 much <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: ark.intel.com 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 readings.
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)
<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 storeio/zero) <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 architecture <antrik> apparently Nehalem is where Intel first introduces nested page tables <antrik> which pretty much explains the considerably lower overhead of VM magic <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 manipulations <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 evaluation <braunr> antrik: do you know if xen benefits from nested page tables ? <antrik> no idea
<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 EPT <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 after <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> http://www.reddit.com/r/gnu/comments/k68mb/how_intelamd_inadvertently_fixed_gnu_hurd/ <sudoman> so is the dd command pointed to by that article a measure of io performance? <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 that... <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.