Re: [PATCH v8 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads

[Jiri Olsa]

On Tue, Sep 11, 2018 at 04:42:09PM +0300, Alexey Budankov wrote:
> Hi,
> 
> On 11.09.2018 11:34, Jiri Olsa wrote:
> > On Tue, Sep 11, 2018 at 11:16:45AM +0300, Alexey Budankov wrote:
> >>
> >> Hi Ingo,
> >>
> >> On 11.09.2018 9:35, Ingo Molnar wrote:
> >>>
> >>> * Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> wrote:
> >>>
> >>>> It may sound too optimistic but glibc API is expected to be backward compatible 
> >>>> and for POSIX AIO API part too. Internal implementation also tends to evolve to 
> >>>> better option overtime, more probably basing on modern kernel capabilities 
> >>>> mentioned here: http://man7.org/linux/man-pages/man2/io_submit.2.html
> >>>
> >>> I'm not talking about compatibility, and I'm not just talking about glibc, perf works under 
> >>> other libcs as well - and let me phrase it in another way: basic event handling, threading, 
> >>> scheduling internals should be a *core competency* of a tracing/profiling tool.
> >>
> >> Well, the requirement of independence from some specific libc implementation 
> >> as well as *core competency* design approach clarify a lot. Thanks!
> >>
> >>>
> >>> I.e. we might end up using the exact same per event fd thread pool design that glibc uses 
> >>> currently. Or not. Having that internal and open coded to perf, like Jiri has started 
> >>> implementing it, allows people to experiment with it.
> >>
> >> My point here is that following some standardized programming models and APIs 
> >> (like POSIX) in the tool code, even if the tool itself provides internal open 
> >> coded implementation for the APIs, would simplify experimenting with the tool 
> >> as well as lower barriers for new comers. Perf project could benefit from that.
> >>
> >>>
> >>> This isn't some GUI toolkit, this is at the essence of perf, and we are not very good on large 
> >>> systems right now, and I think the design should be open-coded threading, not relying on an 
> >>> (perf-)external AIO library to get it right.
> >>>
> >>> The glibc thread pool implementation of POSIX AIO is basically a fall-back 
> >>> implementation, for the case where there's no native KAIO interface to rely on.
> >>>
> >>>> Well, explicit threading in the tool for AIO, in the simplest case, means 
> >>>> incorporating some POSIX API implementation into the tool, avoiding 
> >>>> code reuse in the first place. That tends to be error prone and costly.
> >>>
> >>> It's a core competency, we better do it right and not outsource it.
> >>
> >> Yep, makes sense.
> > 
> > on the other hand, we are already trying to tie this up under perf_mmap
> > object, which is what the threaded patchset operates on.. so I'm quite
> > confident that with little effort we could make those 2 things live next
> > to each other and let the user to decide which one to take and compare
> > 
> > possibilities would be like: (not sure yet the last one makes sense, but still..)
> > 
> >   # perf record --threads=...  ...
> >   # perf record --aio ...
> >   # perf record --threads=... --aio ...
> > 
> > how about that?
> 
> That might be an option. What is the semantics of --threads?

that's my latest post on this:
  https://marc.info/?l=linux-kernel&m=151551213322861&w=2

working on repost ;-)

jirka

> 
> Be aware that when experimenting with serial trace writing on an 8-core 
> client machines running an HPC benchmark heavily utilizing all 8 cores 
> we noticed that single Perf tool thread contended with the benchmark 
> threads.
> 
> That manifested like libiomp.so (Intel OpenMP implementation) functions 
> appearing among the top hotspots functions and this was indication of 
> imbalance induced by the tool during profiling.
> 
> That's why we decided to first go with AIO approach, as it is posted,
> and benefit from it the most thru multi AIO, prior turning to more 
> resource consuming multi-threading alternative. 
> 
> > 
> > I just rebased the thread patchset, will make some tests (it's been few months,
> > so it needs some kicking/checking) and post it out hopefuly this week> 
> > jirka
> >