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Gregg started with a affirmation apparatus that he had aloof written: it’s actual appearance was in the conception of a acute accent that assorted in abundance as he absolved about the lectern. It was, it turns out, a BPF-based apparatus that extracts the arresting backbone of the laptop’s WiFi affiliation from the atom and creates a babble in response. As he interfered with that arresting with his body, the backbone (and appropriately the angle of the tone) varied. By tethering the laptop to his phone, he acclimated the apparatus to admeasurement how abutting he was to the laptop. It may not be the best applied tool, but it did authenticate how BPF can be acclimated to do abrupt things.
Gregg works at Netflix, a aggregation that about operates about 150,000 server instances. Naturally, Netflix cares a lot about performance; that leads to a admiration for observability accoutrement that can advice to define the antecedent of achievement problems. But the amount of acceptable accoutrement goes aloft aloof achievement tuning.
For example, the aggregation is currently aggravating to move its server images advanced to the 4.15 kernel, but a snag has appear up. There is a log-rotation account that waits for a specific activity to avenue by again active ps. It seems like a simple task, but it started declining on the newer kernel; ps would abort to address the activity when, in fact, that activity was still running. It was, Gregg said, time to investigate.
He began with the bpftrace apparatus which, he said, is anon to be packaged by a cardinal of distributors. It could be interesting, he thought, to see which arrangement calls are declining beneath the new atom that formed beneath the old. Award out which arrangement calls are declining can be done with a command like:
This has to attach to all 316 arrangement calls, so it takes a while to get going; there is allocution of a “multi-attach” functionality to acceleration things up in the future. In the meantime, one can get about that botheration by alteration the command to attach to the raw system-call tracepoint instead:
So, perhaps, the botheration lives in the /proc filesystem. Switching to an ftrace-based apparatus for a moment, he looked at which functions were actuality alleged in the bootless runs. Again aback to bpftrace to hunt a few amiss leads afore free that find_ge_pid(), which is declared to abide a apprehend by award the abutting accomplished activity ID, was endlessly early. That, in turn, appears to accept been acquired by a change in the accomplishing of the process-ID table. And at that point he had to stop to biking to the conference.
He appropriately didn’t yet accept a band-aid to the botheration (though, aback the accusable parties were in the room, a band-aid seems acceptable to appear soon). But, he said, alike at this point BPF has helped to decidedly attenuated bottomward the problem. One of the bigger allowances to the accepted crop of BPF-based observability tools, he said, is that they accredit the conception of abundant bigger bug reports.
Stepping aback briefly, Gregg talked about the architectonics of the BPF system. At the everyman akin is the BPF basic apparatus itself. One can address raw BPF cipher for this machine, but it’s “super hard” and cipher should do it. He asked the bodies in the admirers who had done this to accession their easily and was afraid by how abounding easily went up; the allowance contained, he said, the absolute apple citizenry of raw BPF experts. For those absent a higher-level experience, though, an LLVM backend allows BPF programs to be accounting in a belted adaptation of C. Then, the BCC arrangement allows those programs to be loaded and apprenticed from C or Python code. And now, aloft that, is bpftrace, which allows the autograph of advantageous one-line tools.
So, for example, he asked about the capability of readahead in the folio cache: is it absolutely accelerating I/O, or is it aloof communicable the cache? One charge not admiration about such things; they can now be observed. So he wrote a simple apparatus in bpftrace to almanac timestamps for all pages brought in via readahead; it again compares the timestamps aback those pages are absolutely referenced. The result, for his workload, was that about all pages were accessed aural 32ms of actuality apprehend into the cache; the cardinal of pages that were never referenced was absolutely small. So readahead is absolutely allowance here; if anything, the appliance in catechism could account from alike added readahead.
One can ask agnate questions about boot from the folio cache: by attractive at the age of pages as they are pushed out, it is accessible to appraise whether the folio accumulation is appropriately sized. If those pages accept not been referenced in a continued time, conceivably the accumulation is too large. This apparatus does not yet abide but, he suggested, conceivably the accumulated accumulation could put an accomplishing calm during the appointment and accelerate it to him.
The apparatus to acknowledgment the readahead catechism was not absolutely a one-liner, but it fit accurately assimilate a distinct slide. bpftrace, he said, is a abundant belvedere for the conception of abbreviate tools. In its simplest form, one charge alone accommodate a delving (a atom tracepoint, for example), an alternative clarify expression, and an activity to booty aback the delving is hit. The accent overall, he said, looks a lot like awk or DTrace.
Inside, it has a parser that builds an abstruse syntax timberline anecdotic the task, which is again acclimated to actualize the BPF program(s) to attach to the contest of interest. The perf arena absorber is acclimated to get high-bandwidth abstracts out of the atom but, to the greatest admeasurement possible, abstracts is above bottomward in atom amplitude aboriginal and exported via BPF maps. Adaptation 0.90 was appear in March; the abutting release, alleged 0.10, is due anon (though that absolution cardinal may be afflicted afterwards some complaints were aloft about the credible astern cardinal jump).
As Gregg works on a book about achievement assay with BPF, he is aggravating to ample in assorted “observability gaps” in the system. To that end, he brought aback his old superping utility, which absorbed into packet accelerate and accept contest in an attack to access added authentic ping times by eliminating any scheduling latency. But he assured up advertisement best times than accustomed ping does — not the accepted result. It turns out that this botheration has been apparent in the networking assemblage years ago, with timestamps actuality stored in the packets themselves, so this apparatus accepted unnecessary. The superping apparatus demonstrates, though, how BPF programs can assignment from atom headers to airing through chains of structures.
A apparatus that is still useful, instead, is alleged tcplife; it collects advice about TCP access to and from the bounded system. This affectionate of abstracts is commonly generated through packet capture, which is an big-ticket affair to do; tcplife, instead, aloof hooks into the networking assemblage to get contest aback access activate and end. Initially it acclimated a kprobe set on tcp_set_state(); that advice accepted advantageous abundant that a able tracepoint was added in 4.15. That was an improvement; tracepoints are added abiding and beneath decumbent to breaking than kprobes. But alone to a point.
In 4.16, that tracepoint was confused to inet_sock_set_state() and some added advice was added to it; that bankrupt all of Gregg’s accoutrement that were appliance the earlier tracepoint. And, he said, he is “fine with that”. Alike if a tracepoint changes occasionally, it’s far bigger than appliance raw kprobes. He did accept one request, though: tracepoints should accommodate a absolute arrow to the structure(s) in use at that breadth in the code. He realizes that any cipher appliance that arrow will be unstable, but it’s still advantageous to accept if there is a charge for abstracts that is not exported anon by the tracepoint.
This case illustrates a added accepted point, he said: kprobes can serve as a advantageous way to ancestor tracepoints. A apparatus congenital about a kprobe shows what the use case for the tracepoint would be and which abstracts should be included; that helps to absolve the tracepoint’s accession and to get it appropriate aback that happens.
Gregg assured with a “reality check”: alike admitting the assorted BPF accoutrement accommodate all kinds of advantageous information, best achievement wins still appear from poring over blaze graphs instead. But, naturally, BPF is affective into that breadth as well; it is now accessible to actualize trace summaries in the kernel, abbreviation the aerial of accession that array of achievement data. That, he said, shows the administration the atom association should abide to booty in the future: “BPF all the things”.
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