Lin­ux­u­la­tor D‑Trace probes com­mit­ted to cur­rent

A while ago I com­mit­ted the lin­ux­u­la­tor D‑Trace probes I talked about ear­li­er. I wait­ed a lit­tle bit for this announce­ment to make sure I have not bro­ken any­thing. Nobody com­plained so far, so I assume noth­ing obvi­ous­ly bad crept in.

The >500 probes I com­mit­ted do not cov­er the entire lin­ux­u­la­tor, but are a good start. Adding new ones is straight for­ward, if some­one is inter­est­ed in a junior-ker­nel-hack­er task, this would be one. Just ask me (or ask on emu­la­tion@), and I can guide you through it.

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Lin­ux­u­la­tor progress

This week­end I made some progress in the lin­ux­u­la­tor:

  • I MFCed the report­ing of some linux-syscalls to 9‑stable and 8‑stable.
  • I updat­ed my lin­ux­u­la­tor-dtrace patch to a recent ‑cur­rent. I already com­piled it on i386 and arundel@ has it com­piled on amd64. I count­ed more than 500 new DTrace probes. Now that DTrace res­cans for SDT probes when a ker­nel mod­ule is loaded, there is no ker­nel pan­ic any­more when the lin­ux mod­ule is loaded after the DTrace mod­ules and you want to use DTrace. I try to com­mit this at a morn­ing of a day where I can fix things dur­ing the day in case some prob­lems show up which I did not notice dur­ing my test­ing.
  • I cre­at­ed a PR for portmgr@ to repocopy a new linux_base port.
  • I set the expi­ra­tion date of linux_base-fc4 (only used by 7.x and upstream way past its EoL) and all depen­dent ports. It is set to the EoL of the last 7.x release, which can not use a lat­er linux_base port. I also added a com­ment which explains that the date is the EoL of the last 7.x release.
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DTrace probes for the Lin­ux­u­la­tor updat­ed

If some­one had a look at the ear­li­er post about DTrace probes for the Lin­ux­u­la­tor: I updat­ed the patch at the same place. The dif­fer­ence between the pre­vi­ous one is that some D-scripts are fixed now to do what I meant, spe­cial­ly the ones which pro­vide sta­tis­tics out­put.

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New DTrace probes for the lin­ux­u­la­tor

I for­ward port­ed my DTrace probes for the FreeB­SD lin­ux­u­la­tor from a 2008-current to a recent ‑cur­rent. I have not the com­plete FreeB­SD lin­ux­u­la­tor cov­ered, but a big part is already done. I can check the major locks in the lin­ux­u­la­tor, trace futex­es, and I have a D‑script which yells at a lot of errors which could hap­pen but should not.

Some of my D‑scripts need some changes, as real-world test­ing showed that they are not real­ly work­ing as expect­ed. They can get over­whelmed by the amount of spec­u­la­tion and dynam­ic vari­ables (error mes­sage: dynam­ic vari­able drops with non-empty dirty list). For the dynam­ic vari­ables prob­lem I found a dis­cus­sion on the net with some sug­ges­tions. For the spec­u­la­tion part I expect sim­i­lar tuning-possibilities.

Unfor­tu­nate­ly the D‑script which checks the inter­nal locks fails to com­pile. Seems there is a lit­tle mis­un­der­stand­ing on my side how the D‑language is sup­posed to work.

I try to get some time lat­er to have a look at those prob­lems.

Dur­ing my devel­op­ment I stum­bled over some gener­ic DTrace prob­lems with the SDT provider I use for my probes:

  • If you load the Lin­ux mod­ule after the SDT mod­ule, your sys­tem will pan­ic as soon as you want to access some probes, e.g. “dtrace ‑l” will pan­ic the sys­tem. Load­ing the Lin­ux mod­ule before the SDT mod­ule pre­vents the pan­ic.
  • Unload­ing the SDT mod­ule while the Lin­ux mod­ule with the SDT probes is still loaded pan­ics the sys­tem too. Do not unload the Lin­ux mod­ule if you run with my patch.

Accord­ing to avg@ those are known prob­lems, but I think nobody is work­ing on this. This is bad, because this means I can not com­mit my cur­rent patch­set.

If some­one wants to try the new DTrace probes for the lin­ux­u­la­tor, feel free to go to http://www.Leidinger.net/FreeBSD/current-patches/ and down­load linuxulator-dtrace.diff. I do not offer a work­ing hyper­link here on pur­pose, the SDT bugs can hurt if you are not care­ful, and I want to make the use of this patch a strong opt-in because of this. If the patch hurts you, it is your fault, you have been warned.

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Under­stand­ing laten­cy

Bren­dan Gregg of Sun Ora­cle fame made a good expla­na­tion how to visu­al­ize laten­cy to get a bet­ter under­stand­ing of what is going on (and as such about how to solve bot­tle­necks). I have seen all this already in var­i­ous posts in his blog and in the Ana­lyt­ics pack­age in an Open­Stor­age pre­sen­ta­tion, but the ACM arti­cle sum­ma­rizes it very good.

Unfor­tu­nate­ly Ana­lyt­ics is AFAIK not avail­able in Open­So­laris, so we can not go out and adapt it for FreeB­SD (which would prob­a­bly require to port/implement some addi­tion­al dtrace stuff/probes). I am sure some­thing like this would be very inter­est­ing to all those com­pa­nies which use FreeB­SD in an appli­ance (regard­less if it is a stor­age appli­ance like NetApp, or a net­work appli­ance like a Cis­co/Juniper router, or any­thing else which has to per­form good).

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