> -----Original Message----- > From: Wiles, Keith > Sent: Wednesday, August 23, 2017 11:50 AM > To: Carrillo, Erik G <erik.g.carri...@intel.com> > Cc: rsanf...@akamai.com; dev@dpdk.org > Subject: Re: [dpdk-dev] [PATCH 0/3] *** timer library enhancements *** > > > > On Aug 23, 2017, at 11:19 AM, Carrillo, Erik G <erik.g.carri...@intel.com> > wrote: > > > > > > > >> -----Original Message----- > >> From: Wiles, Keith > >> Sent: Wednesday, August 23, 2017 10:02 AM > >> To: Carrillo, Erik G <erik.g.carri...@intel.com> > >> Cc: rsanf...@akamai.com; dev@dpdk.org > >> Subject: Re: [dpdk-dev] [PATCH 0/3] *** timer library enhancements > >> *** > >> > >> > >>> On Aug 23, 2017, at 9:47 AM, Gabriel Carrillo > >>> <erik.g.carri...@intel.com> > >> wrote: > >>> > >>> In the current implementation of the DPDK timer library, timers can > >>> be created and set to be handled by a target lcore by adding it to a > >>> skiplist that corresponds to that lcore. However, if an application > >>> enables multiple lcores, and each of these lcores repeatedly > >>> attempts to install timers on the same target lcore, overall > >>> application throughput will be reduced as all lcores contend to > >>> acquire the lock guarding the single skiplist of pending timers. > >>> > >>> This patchset addresses this scenario by adding an array of > >>> skiplists to each lcore's priv_timer struct, such that when lcore i > >>> installs a timer on lcore k, the timer will be added to the ith > >>> skiplist for lcore k. If lcore j installs a timer on lcore k > >>> simultaneously, lcores i and j can both proceed since they will be > >>> acquiring different locks for different lists. > >>> > >>> When lcore k processes its pending timers, it will traverse each > >>> skiplist in its array and acquire a skiplist's lock while a run list > >>> is broken out; meanwhile, all other lists can continue to be modified. > >>> Then, all run lists for lcore k are collected and traversed together > >>> so timers are executed in their global order. > >> > >> What is the performance and/or latency added to the timeout now? > >> > >> I worry about the case when just about all of the cores are enabled, > >> which could be as high was 128 or more now. > > > > There is a case in the timer_perf_autotest that runs rte_timer_manage > with zero timers that can give a sense of the added latency. When run with > one lcore, it completes in around 25 cycles. When run with 43 lcores (the > highest I have access to at the moment), rte_timer_mange completes in > around 155 cycles. So it looks like each added lcore adds around 3 cycles of > overhead for checking empty lists in my testing. > > Does this mean we have only 25 cycles on the current design or is the 25 > cycles for the new design? >
Both - when run with one lcore, the new design becomes equivalent to the original one. I tested the current design to confirm. > If for the new design, then what is the old design cost compared to the new > cost. > > I also think we need the call to a timer function in the calculation, just to > make sure we have at least one timer in the list and we account for any short > cuts in the code for no timers active. > Looking at the numbers for non-empty lists in timer_perf_autotest, the overhead appears to fall away. Here are some representative runs for timer_perf_autotest: 43 lcores enabled, installing 1M timers on an lcore and processing them with current design: <...snipped...> Appending 1000000 timers Time for 1000000 timers: 424066294 (193ms), Time per timer: 424 (0us) Time for 1000000 callbacks: 73124504 (33ms), Time per callback: 73 (0us) Resetting 1000000 timers Time for 1000000 timers: 1406756396 (641ms), Time per timer: 1406 (1us) <...snipped...> 43 lcores enabled, installing 1M timers on an lcore and processing them with proposed design: <...snipped...> Appending 1000000 timers Time for 1000000 timers: 382912762 (174ms), Time per timer: 382 (0us) Time for 1000000 callbacks: 79194418 (36ms), Time per callback: 79 (0us) Resetting 1000000 timers Time for 1000000 timers: 1427189116 (650ms), Time per timer: 1427 (1us) <...snipped...> The above are not averages, so the numbers don't really indicate which is faster, but they show that the overhead of the proposed design should not be appreciable. > > > >> > >> One option is to have the lcore j that wants to install a timer on > >> lcore k to pass a message via a ring to lcore k to add that timer. We > >> could even add that logic into setting a timer on a different lcore > >> then the caller in the current API. The ring would be a multi-producer and > single consumer, we still have the lock. > >> What am I missing here? > >> > > > > I did try this approach: initially I had a multi-producer single-consumer > > ring > that would hold requests to add or delete a timer from lcore k's skiplist, > but it > didn't really give an appreciable increase in my test application throughput. > In profiling this solution, the hotspot had moved from acquiring the > skiplist's > spinlock to the rte_atomic32_cmpset that the multiple-producer ring code > uses to manipulate the head pointer. > > > > Then, I tried multiple single-producer single-consumer rings per target > lcore. This removed the ring hotspot, but the performance didn't increase as > much as with the proposed solution. These solutions also add overhead to > rte_timer_manage, as it would have to process the rings and then process > the skiplists. > > > > One other thing to note is that a solution that uses such messages changes > the use models for the timer. One interesting example is: > > - lcore I enqueues a message to install a timer on lcore k > > - lcore k runs rte_timer_manage, processes its messages and adds the > > timer to its list > > - lcore I then enqueues a message to stop the same timer, now owned by > > lcore k > > - lcore k does not run rte_timer_manage again > > - lcore I wants to free the timer but it might not be safe > > This case seems like a mistake to me as lcore k should continue to call > rte_timer_manager() to process any new timers from other lcores not just > the case where the list becomes empty and lcore k does not add timer to his > list. > > > > > Even though lcore I has successfully enqueued the request to stop the > timer (and delete it from lcore k's pending list), it hasn't actually been > deleted from the list yet, so freeing it could corrupt the list. This case > exists > in the existing timer stress tests. > > > > Another interesting scenario is: > > - lcore I resets a timer to install it on lcore k > > - lcore j resets the same timer to install it on lcore k > > - then, lcore k runs timer_manage > > This one also seems like a mistake, more then one lcore setting the same > timer seems like a problem and should not be done. A lcore should own a > timer and no other lcore should be able to change that timer. If multiple > lcores need a timer then they should not share the same timer structure. > Both of the above cases exist in the timer library stress tests, so a solution would presumably need to address them or it would be less flexible. The original design passed these tests, as does the proposed one. > > > > Lcore j's message obviates lcore i's message, and it would be wasted work > for lcore k to process it, so we should mark it to be skipped over. > Handling all > the edge cases was more complex than the solution proposed. > > Hmmm, to me it seems simple here as long as the lcores follow the same > rules and sharing a timer structure is very risky and avoidable IMO. > > Once you have lcores adding timers to another lcore then all accesses to that > skip list must be serialized or you get unpredictable results. This should > also > fix most of the edge cases you are talking about. > > Also it seems to me the case with an lcore adding timers to another lcore > timer list is a specific use case and could be handled by a different set of > APIs > for that specific use case. Then we do not need to change the current design > and all of the overhead is placed on the new APIs/design. IMO we are > turning the current timer design into a global timer design as it really is a > per > lcore design today and I beleive that is a mistake. > Well, the original API explicitly supports installing a timer to be executed on a different lcore, and there are no API changes in the patchset. Also, the proposed design keeps the per-lcore design intact; it only takes what used to be one large skiplist that held timers for all installing lcores, and separates it into N skiplists that correspond 1:1 to an installing lcore. When an lcore processes timers on its lists it will still only be managing timers it owns, and no others. > > > >>> > >>> Gabriel Carrillo (3): > >>> timer: add per-installer pending lists for each lcore > >>> timer: handle timers installed from non-EAL threads > >>> doc: update timer lib docs > >>> > >>> doc/guides/prog_guide/timer_lib.rst | 19 ++- > >>> lib/librte_timer/rte_timer.c | 329 +++++++++++++++++++++++------ > --- > >> ---- > >>> lib/librte_timer/rte_timer.h | 9 +- > >>> 3 files changed, 231 insertions(+), 126 deletions(-) > >>> > >>> -- > >>> 2.6.4 > >>> > >> > >> Regards, > >> Keith > > Regards, > Keith
