[dpdk-dev] [PATCH v1] doc: prog guide update for eal multi-pthread

[Butler, Siobhan A]

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Cunming Liang
> Sent: Monday, February 16, 2015 7:34 AM
> To: dev at dpdk.org
> Subject: [dpdk-dev] [PATCH v1] doc: prog guide update for eal multi-pthread
> 
> The patch add the multi-pthread section under EAL chapter of prog_guide.
> 
> Signed-off-by: Cunming Liang <cunming.liang at intel.com>
> ---
>  doc/guides/prog_guide/env_abstraction_layer.rst | 157
> ++++++++++++++++++++++++
>  1 file changed, 157 insertions(+)
> 
> diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst
> b/doc/guides/prog_guide/env_abstraction_layer.rst
> index 231e266..06bcfae 100644
> --- a/doc/guides/prog_guide/env_abstraction_layer.rst
> +++ b/doc/guides/prog_guide/env_abstraction_layer.rst
> @@ -212,4 +212,161 @@ Memory zones can be reserved with specific start
> address alignment by supplying  The alignment value should be a power of
> two and not less than the cache line size (64 bytes).
>  Memory zones can also be reserved from either 2 MB or 1 GB hugepages,
> provided that both are available on the system.
> 
> +
> +Multiple pthread
> +----------------
> +
> +DPDK usually pin one pthread per core to avoid task switch overhead. It
> +gains performance a lot, but it's not flexible and not always efficient.
> +
> +Power management helps to improve the cpu efficient by limiting the cpu
> runtime frequency.
> +But there's more reasonable motivation to utilize the ineffective idle cycles
> under the full capability of cpu.
> +
> +By OS scheduing and cgroup, to each pthread on specified cpu, it can simply
> assign the cpu quota.
> +It gives another way to improve the cpu efficiency. But the prerequisite is to
> run DPDK execution conext from multiple pthread on one core.
> +
> +For flexibility, it's also useful to allow the pthread affinity not only to a cpu
> but to a cpu set.
> +
> +
> +EAL pthread and lcore Affinity
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +In terms of lcore, it stands for an EAL execution unit in the EAL pthread.
> +EAL pthread indicates all the pthreads created/managed by EAL, they
> execute the tasks issued by *remote_launch*.
> +In each EAL pthread, there's a TLS called *_lcore_id* for the unique
> identification.
> +As EAL pthreads usually 1:1 bind to the physical cpu, *_lcore_id* typically
> equals to the cpu id.
> +
> +In multiple pthread case, EAL pthread is no longer always bind to one
> specific physical cpu.
> +It may affinity to a cpuset. Then the *_lcore_id* won't always be the same
> as cpu id.
> +So there's an EAL long option '--lcores' defined to assign the cpu affinity of
> lcores.
> +For a specified lcore id or id group, it allows to set the cpuset for that EAL
> pthread.
> +
> +The format pattern:
> +	--lcores='<lcore_set>[@cpu_set][,<lcore_set>[@cpu_set],...]'
> +
> +'lcore_set' and 'cpu_set' can be a single number, range or a group.
> +
> +A number is a "digit([0-9]+)"; a range is "<number>-<number>"; a group is
> "(<number|range>[,<number|range>,...])".
> +
> +If not supply a '\@cpu_set', the value of 'cpu_set' uses the same value as
> 'lcore_set'.
> +
> +    ::
> +
> +    	For example, "--lcores='1,2@(5-7),(3-5)@(0,2),(0,6),7-8'" which
> means start 9 EAL thread;
> +    	    lcore 0 runs on cpuset 0x41 (cpu 0,6);
> +    	    lcore 1 runs on cpuset 0x2 (cpu 1);
> +    	    lcore 2 runs on cpuset 0xe0 (cpu 5,6,7);
> +    	    lcore 3,4,5 runs on cpuset 0x5 (cpu 0,2);
> +    	    lcore 6 runs on cpuset 0x41 (cpu 0,6);
> +    	    lcore 7 runs on cpuset 0x80 (cpu 7);
> +    	    lcore 8 runs on cpuset 0x100 (cpu 8).
> +
> +By this option, for each given lcore id, the associated cpus can be assigned.
> +It's also compatible with the pattern of corelist('-l') option.
> +
> +non-EAL pthread support
> +~~~~~~~~~~~~~~~~~~~~~~~
> +
> +It allows to use DPDK execution context in any user pthread(aka. non-EAL
> pthread).
> +
> +In a non-EAL pthread, the *_lcore_id* is always LCORE_ID_ANY which
> means it's not an EAL thread along with a valid *_lcore_id*.
> +Then the libraries won't take *_lcore_id* as unique id. Instead of it,
> +some libraries use another alternative unique id(e.g. tid); some are totaly
> no impact; and some work with some limitation(e.g. timer, mempool).
> +
> +All these impacts are mentioned in :ref:`known_issue_label` section.
> +
> +Public Thread API
> +~~~~~~~~~~~~~~~~~
> +
> +There are two public API ``rte_thread_set_affinity()`` and
> ``rte_pthread_get_affinity()`` introduced for threads.
> +When they're used in any pthread context, the Thread Local Storage(TLS)
> will be set/get.
> +
> +Those TLS include *_cpuset* and *_socket_id*:
> +
> +*	*_cpuset* stores the cpus bitmap to which the pthread affinity.
> +
> +*	*_socket_id* stores the NUMA node of the cpuset. If the cpus in
> cpuset belong to different NUMA node, the *_socket_id* set to
> SOCKTE_ID_ANY.
> +
> +
> +.. _known_issue_label:
> +
> +Known Issues
> +~~~~~~~~~~~~
> +
> ++ rte_mempool
> +
> +  The rte_mempool uses a per-lcore cache inside mempool.
> +  For non-EAL pthread, ``rte_lcore_id()`` will not return a valid number.
> +  So for now, when rte_mempool is used in non-EAL pthread, the put/get
> operations will bypass the mempool cache.
> +  There's performance penalty if bypassing the mempool cache. The work
> for none-EAL mempool cache support is in progress.
> +
> +  However, there's another problem. The rte_mempool is not preemptable.
> This comes from rte_ring.
> +
> ++ rte_ring
> +
> +  rte_ring supports multi-producer enqueue and multi-consumer dequeue.
> But it's non-preemptive.
> +
> +  .. note::
> +
> +    The "non-preemptive" constraint means:
> +
> +    - a pthread doing multi-producers enqueues on a given ring must not
> +      be preempted by another pthread doing a multi-producer enqueue on
> +      the same ring.
> +    - a pthread doing multi-consumers dequeues on a given ring must not
> +      be preempted by another pthread doing a multi-consumer dequeue on
> +      the same ring.
> +
> +    Bypassing this constraints may cause the 2nd pthread to spin until the 1st
> one is scheduled again.
> +    Moreover, if the 1st pthread is preempted by a context that has an higher
> priority, it may even cause a dead lock.
> +
> +  But it doesn't means we can't use. Just need to narrow down the situation
> when it's used by multi-pthread on the same core.
> +
> +  1. It CAN be used for any single-producer or single-consumer situation.
> +
> +  2. It MAY be used by multi-producer/consumer pthread whose scheduling
> policy are all SCHED_OTHER(cfs). User SHOULD aware of the performance
> penalty befor using it.
> +
> +  3. It MUST not be used by multi-producer/consumer pthread, while some
> of their scheduling policies is SCHED_FIFO or SCHED_RR.
> +
> +  ``RTE_RING_PAUSE_REP_COUNT`` is defined for rte_ring to reduce
> contention. It's mainly for case 2, a yield is issued after number of times
> pause repeat.
> +
> +  It adds a sched_yield() syscall if the thread spins for too long, waiting other
> thread to finish its operations on the ring.
> +  That gives pre-empted thread a chance to proceed and finish with ring
> enqnue/dequeue operation.
> +
> ++ rte_timer
> +
> +  It's not allowed to run ``rte_timer_manager()`` on a non-EAL pthread. But
> it's all right to reset/stop the timer from a non-EAL pthread.
> +
> ++ rte_log
> +
> +  In non-EAL pthread, there's no per thread loglevel and logtype. It uses the
> global loglevel.
> +
> ++ misc
> +
> +  The debug statistics of rte_ring, rte_mempool and rte_timer are not
> suppoted in a non-EAL pthread.
> +
> +cgroup control
> +~~~~~~~~~~~~~~
> +
> +Here's a simple example, there's two pthreads(t0 and t1) doing packet IO
> on the same core($cpu).
> +We expect only 50% of CPU spend on packet IO.
> +
> +  .. code::
> +
> +    mkdir /sys/fs/cgroup/cpu/pkt_io
> +    mkdir /sys/fs/cgroup/cpuset/pkt_io
> +
> +    echo $cpu > /sys/fs/cgroup/cpuset/cpuset.cpus
> +
> +    echo $t0 > /sys/fs/cgroup/cpu/pkt_io/tasks
> +    echo $t0 > /sys/fs/cgroup/cpuset/pkt_io/tasks
> +
> +    echo $t1 > /sys/fs/cgroup/cpu/pkt_io/tasks
> +    echo $t1 > /sys/fs/cgroup/cpuset/pkt_io/tasks
> +
> +    cd /sys/fs/cgroup/cpu/pkt_io
> +    echo 100000 > pkt_io/cpu.cfs_period_us
> +    echo  50000 > pkt_io/cpu.cfs_quota_us
> +
> +
>  .. |linuxapp_launch| image:: img/linuxapp_launch.svg
> --
> 1.8.1.4

Acked-by: Siobhan Butler <siobhan.a.butler at intel.com>