Re: [PATCH 1/2] perf core: Add a kmem_cache for struct perf_event

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 11, 2021 at 08:54:12PM +0900, Namhyung Kim wrote:
> From: Namhyung Kim <namhyung@google.com>
> 
> The kernel can allocate a lot of struct perf_event when profiling. For
> example, 256 cpu x 8 events x 20 cgroups = 40K instances of the struct
> would be allocated on a large system.
> 
> The size of struct perf_event in my setup is 1152 byte. As it's
> allocated by kmalloc, the actual allocation size would be rounded up
> to 2K.
> 
> Then there's 896 byte (~43%) of waste per instance resulting in total
> ~35MB with 40K instances. We can create a dedicated kmem_cache to
> avoid such a big unnecessary memory consumption.
> 
> With this change, I can see below (note this machine has 112 cpus).
> 
>   # grep perf_event /proc/slabinfo
>   perf_event    224    784   1152    7    2 : tunables   24   12    8 : slabdata    112    112      0
> 
> The sixth column is pages-per-slab which is 2, and the fifth column is
> obj-per-slab which is 7.  Thus actually it can use 1152 x 7 = 8064
> byte in the 8K, and wasted memory is (8192 - 8064) / 7 = ~18 byte per
> instance.
> 
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Thanks for both!