Messages in this thread | | | Date | Mon, 11 Jan 2016 22:00:08 -0800 | From | Alexei Starovoitov <> | Subject | Re: [PATCH net-next 0/4] bpf: bpf_htab: Add BPF_MAP_TYPE_PERCPU_HASH |
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On Tue, Jan 12, 2016 at 01:48:10PM +0800, Ming Lei wrote: > The total memory consumption is still much less than memory consumed > by percpu hash since a new element is only added to hash if the key is run > on that CPU. Most of times, for one key it may touch very few CPUs. > > For percpu hash, the memory is always allocated to every CPU no matter > if the key is run from the CPU.
In Martin's use case all cpus are servicing network traffic and all of them are counting packets.
> In my test, removing the current kmalloc() in update element callback can > improve io thoughput by 10% not mention the percpu ida allocation cost, and > looks it isn't cheap. That is why I don't think it is good to > introduce another new > allocation in the eBPF prog path.
I don't think anyone is arguing that pre-allocation is not needed. In some cases better performance can be achieved with pre-allocation, in some other cases regular hash map will be enough, and in others hash map with per-cpu is needed as well.
> You can find my test in the link below: > > https://patchwork.ozlabs.org/patch/556926/
yes, for tools/biolatency pre-allocation is a win, but in many other cases we simply cannot pre-allocate all elements.
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