On 10/15/13 1:44 AM, Ingo Molnar wrote:
* Namhyung Kim <[email protected]> wrote:
[SNIP]
+/* mmap file big chunks at a time */
+#define MMAP_OUTPUT_SIZE (64*1024*1024)
Why did you choose 64MB for the size? Did you also test other sizes?
Btw., should this value go up if the ring buffer (mmap_pages) is larger
than 64MB?
I made mmap_size a variable:
+ size_t mmap_size; /* size of mmap segments */
with the above initial value. I was planning to make it in an option and
just forgot to complete it.
Why 64M? mmap / munmap are also system calls and I was looking to trade
off huge file size jumps versus the frequency of adjusting the maps. 64M
Was just a nice round number between 1 and 100. 8, 16 are too small. 128
seems to big for a default. That left only 32 and 64. 64M seems the
better trade off of the two.
Making it a user knob would help with smaller deployments. Could also
have mmap_size = 0 mean turn it off (use write over mmap).
Perhaps something that adjust automatically would be useful too. e.g.,
For the case that motivates the change I have 16 cpus each with a 4M
buffer (1024 mmap pages). Should we generically set the size:
mmap_size = ncpus_online * mmap_pages * page_size?
Do that only for system wide profiling?
David
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