Re: [PATCH 5/6] bpf: hash: avoid to call kmalloc() in eBPF prog

[Alexei Starovoitov]

On Tue, Dec 15, 2015 at 07:21:03PM +0800, Ming Lei wrote:
> kmalloc() is often a bit time-consuming, also
> one atomic counter has to be used to track the total
> allocated elements, which is also not good.
> 
> This patch pre-allocates element pool in htab_map_alloc(),
> then use percpu_ida to allocate one slot from the pool,
> then the runtime allocation/freeing cost can be decreased.
> 
> From my test, at least 10% fio throughput is improved in block
> I/O test when tools/biolatency of bcc(iovisor) is running.
> 
> Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>

Looks very intersting as well.
Approach looks good.
If you can make a common allocation helper for this map and
for blk-mq would be even better.

> -	htab->elem_size = sizeof(struct htab_elem) +
> -			  round_up(htab->map.key_size, 8) +
> -			  htab->map.value_size;
> +	htab->elem_size = round_up(sizeof(struct htab_elem) +
> +				   round_up(htab->map.key_size, 8) +
> +				   htab->map.value_size,
> +				   cache_line_size());

this rounding to cache line is great for performance, but it's extra
memory upfront which may not be needed. The per-allocation is a classic
performance vs memory trade-off. In other cases it may hurt.
So could you change the patch to do pre-allocation only when
requested by user space via extra flag for hash map or via
new BPF_MAP_TYPE_HASH_PREALLOC type? Not sure yet whether flag or
new type is better. I guess implementation will dictate.

PS
Glad that you found iovisor/tools/biolatency useful.
It's indeed pretty helpful to analyze real-time block io latency.

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