From: "Valdis Klētnieks" <valdis.kletnieks@vt.edu>
To: Theodore Dubois <tbodt@google.com>
Cc: kernelnewbies@kernelnewbies.org, a.p.zijlstra@chello.nl,
linux-kernel@vger.kernel.org
Subject: Re: perf_event wakeup_events = 0
Date: Sat, 07 Sep 2019 18:45:31 -0400 [thread overview]
Message-ID: <972489.1567896331@turing-police> (raw)
In-Reply-To: <123C743E-C322-45DB-8796-BF6B6EE9CA80@google.com>
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On Sat, 07 Sep 2019 09:14:49 -0700, Theodore Dubois said:
Reading what it actually says rather than what I thought it said.. :)
Events come in two flavors: counting and sampled. A counting event is
one that is used for counting the aggregate number of events that
occur. In general, counting event results are gathered with a read(2)
call. A sampling event periodically writes measurements to a buffer
that can then be accessed via mmap(2).
For some reason, I was thinking counting events. -ENOCAFFEINE. :)
> sample_freq is 4000 (and freq is 1). Here’s the man page on this field:
>
> sample_period, sample_freq
> A "sampling" event is one that generates an overflow notifica‐
> tion every N events, where N is given by sample_period. A sam‐
> pling event has sample_period > 0.
There's this part:
> pling event has sample_period > 0. When an overflow occurs,
> requested data is recorded in the mmap buffer. The sample_type
> field controls what data is recorded on each overflow.
So an entry is made in the buffer. It's not clear that this immediately triggers
a signal...
MMAP layout
When using perf_event_open() in sampled mode, asynchronous events (like
counter overflow or PROT_EXEC mmap tracking) are logged into a ring-
buffer. This ring-buffer is created and accessed through mmap(2).
The mmap size should be 1+2^n pages, where the first page is a metadata
page (struct perf_event_mmap_page) that contains various bits of infor?
mation such as where the ring-buffer head is.
So you need to look at what size mmap buffer is being allocated. It's *probably*
on the order of megabytes, so that you can buffer a fairly large number of entries
and not take several user/kernel transitions on every single entry...
> If I’m reading this right, this is a sampling event which overflows 4000 times a second.
And 4,000 entries are made in the buffer per second..
> But perf then does a poll call which wakes up on this FD with POLLIN after
> 1.637 seconds, instead of 0.00025 seconds
At which point perf goes and looks at several thousand entries in the ring buffer...
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next prev parent reply other threads:[~2019-09-07 22:46 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-06 23:28 perf_event wakeup_events = 0 Theodore Dubois
2019-09-07 13:40 ` Valdis Klētnieks
2019-09-07 16:14 ` Theodore Dubois
2019-09-07 22:00 ` Valdis Klētnieks
2019-09-07 22:45 ` Valdis Klētnieks [this message]
2019-09-07 23:27 ` Theodore Dubois
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