Re: [PATCH v2 06/15] perf session: load data directory into tool process memory

[Alexey Budankov]

On 24.10.2020 18:43, Jiri Olsa wrote:
> On Wed, Oct 21, 2020 at 07:01:19PM +0300, Alexey Budankov wrote:
>>
>> Read trace files located in data directory into tool process memory.
>> Basic analysis support of data directories is provided for report
>> mode. Raw dump (-D) and aggregated reports are available for data
>> directories, still with no memory consumption optimizations. However
>> data directories collected with --compression-level option enabled
>> can be analyzed with little less memory because trace files are
>> unmaped from tool process memory after loading collected data.
>> The implementation is based on the prototype [1], [2].
>>
>> [1] git clone https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git -b perf/record_threads
>> [2] https://lore.kernel.org/lkml/20180913125450.21342-1-jolsa@xxxxxxxxxx/
>>
>> Suggested-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> 
> very loosely ;-) so there was a reason for all that reader refactoring,
> so we could have __perf_session__process_dir_events function:
> 
>   https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/commit/?h=perf/record_threads&id=308aa7cff1fed335401cfc02c7bac1a4644af68e

Nonetheless. All that are necessary parts to make threaded data streaming
and analysis eventually merged into the mainline as joint Perf developers
community effort.

> 
> when reporting the threaded record data on really big servers,
> you will run out of memory, so you need to read and flush all
> the files together by smaller pieces

Yes, handling all that _big_ data after collection to make it
helpful for analysis of performance issues is the other part
of this story so that possible OOM should be somehow avoided.

> 
> IMO we need to have this change before we allow threaded record

There are use cases of perf tool as a data provider, btw VTune is not
the only one of them, and for those use cases threaded trace streaming
lets its users get to their data that the users just were loosing before.
This is huge difference and whole new level of support for such users.
Post-process scripting around perf (e.g. Python based) will benefit
from threaded trace streaming. Pipe mode can be extended to stream into
open and passed fds using threads (e.g. perf record -o -fd:13,14,15,16).
VTune-like tools can get performance data, load it into a (relational)
DB files and provide analysis. And all that uses perf tool at its core.

I agree perf report OOM issue can exist on really-big servers but data 
directories support for report mode for not-so-big servers and desktops
is already enabled with this smaller change. Also really-big-servers
come with really-big amount of memory and collection could possibly be
limited to only interesting phases of execution so the issue could likely
be avoided. At the same time threaded trace streaming could clarify on
real use cases that are blocked by perf report OOM issue and that would
clarify on exact required solution. So perf report OOM issue shouldn't
be the showstopper for upstream of threaded trace streaming.

Alexei