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). Hereâ??s the man page on this field:
>
> sample_period, sample_freq
> A "sampling" event is one that generates an overflow notificaâ??
> tion every N events, where N is given by sample_period. A samâ??
> 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 Iâ??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...
Attachment:
pgp0OypdzX6dJ.pgp
Description: PGP signature
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
