Re: I.10 - Event buffer minimal useful size

[Ingo Molnar]

> 10/ Event buffer minimal useful size
>
> As it stands, the buffer header occupies the first page, even
> though the buffer header struct is 32-byte long. That's a lot of
> precious RLIMIT_MEMLOCK memory wasted.
>
>  The actual buffer (data) starts at the next page (from builtin-top.c):
>
> static void mmap_read_counter(struct mmap_data *md)
> {
>	unsigned int head = mmap_read_head(md);
>	unsigned int old = md->prev;
>	unsigned char *data = md->base + page_size;
>
> Given that the buffer "full" notification are sent on page
> crossing boundaries, if the actual buffer payload size is 1 page,
> you are guaranteed to have your samples overwritten.
>
> This leads me to believe that the minimal buffer size to get
> useful data is 3 pages. This is per event group per thread. That
> puts a lot of pressure on RLIMIT_MEMLOCK which is ususally set
> fairly low by distros.

Regarding notifications: you can actually tell it to generate
wakeups every 'n' events instead of at page boundaries. (See:
attr.wakeup_events).

Regarding locked pags: there is an extra per user mlock limit for
perf counters found in sysctl_perf_counter_mlock so it can be
configured independently. Furthermore, the accounting is done on a
per struct user basis, not on a per task basis - which makes the use
of locked pages far less of an issue to distros.

Regarding using 4K for the sampling data header: we are making use
of that property. We recently extended the header to include more
fields and having it writable - so the 4K MMU separation between
data header and data pages very much paid off already - and it could
provide more advantages in the future.
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