> From: Olivier Matz [mailto:olivier.m...@6wind.com] > Sent: Thursday, 27 October 2022 10.35 > > Hi Morten, > > On Wed, Oct 26, 2022 at 04:44:36PM +0200, Morten Brørup wrote: > > Add __rte_cache_aligned to the objs array. > > > > It makes no difference in the general case, but if get/put operations > are > > always 32 objects, it will reduce the number of memory (or last level > > cache) accesses from five to four 64 B cache lines for every get/put > > operation. > > > > For readability reasons, an example using 16 objects follows: > > > > Currently, with 16 objects (128B), we access to 3 > > cache lines: > > > > ┌────────┐ > > │len │ > > cache │********│--- > > line0 │********│ ^ > > │********│ | > > ├────────┤ | 16 objects > > │********│ | 128B > > cache │********│ | > > line1 │********│ | > > │********│ | > > ├────────┤ | > > │********│_v_ > > cache │ │ > > line2 │ │ > > │ │ > > └────────┘ > > > > With the alignment, it is also 3 cache lines: > > > > ┌────────┐ > > │len │ > > cache │ │ > > line0 │ │ > > │ │ > > ├────────┤--- > > │********│ ^ > > cache │********│ | > > line1 │********│ | > > │********│ | > > ├────────┤ | 16 objects > > │********│ | 128B > > cache │********│ | > > line2 │********│ | > > │********│ v > > └────────┘--- > > > > However, accessing the objects at the bottom of the mempool cache is > a > > special case, where cache line0 is also used for objects. > > > > Consider the next burst (and any following bursts): > > > > Current: > > ┌────────┐ > > │len │ > > cache │ │ > > line0 │ │ > > │ │ > > ├────────┤ > > │ │ > > cache │ │ > > line1 │ │ > > │ │ > > ├────────┤ > > │ │ > > cache │********│--- > > line2 │********│ ^ > > │********│ | > > ├────────┤ | 16 objects > > │********│ | 128B > > cache │********│ | > > line3 │********│ | > > │********│ | > > ├────────┤ | > > │********│_v_ > > cache │ │ > > line4 │ │ > > │ │ > > └────────┘ > > 4 cache lines touched, incl. line0 for len. > > > > With the proposed alignment: > > ┌────────┐ > > │len │ > > cache │ │ > > line0 │ │ > > │ │ > > ├────────┤ > > │ │ > > cache │ │ > > line1 │ │ > > │ │ > > ├────────┤ > > │ │ > > cache │ │ > > line2 │ │ > > │ │ > > ├────────┤ > > │********│--- > > cache │********│ ^ > > line3 │********│ | > > │********│ | 16 objects > > ├────────┤ | 128B > > │********│ | > > cache │********│ | > > line4 │********│ | > > │********│_v_ > > └────────┘ > > Only 3 cache lines touched, incl. line0 for len. > > I understand your logic, but are we sure that having an application > that > works with bulks of 32 means that the cache will stay aligned to 32 > elements for the whole life of the application? > > In an application, the alignment of the cache can change if you have > any of: > - software queues (reassembly for instance) > - packet duplication (bridge, multicast) > - locally generated packets (keepalive, control protocol) > - pipeline to other cores > > Even with testpmd, which work by bulk of 32, I can see that the size > of the cache filling is not aligned to 32. Right after starting the > application, we already have this: > > internal cache infos: > cache_size=250 > cache_count[0]=231 > > This is probably related to the hw rx rings size, number of queues, > number of ports. > > The "250" default value for cache size in testpmd is questionable, but > with --mbcache=256, the behavior is similar. > > Also, when we transmit to a NIC, the mbufs are not returned immediatly > to the pool, they may stay in the hw tx ring during some time, which is > a driver decision. > > After processing traffic on cores 8 and 24 with this testpmd, I get: > cache_count[0]=231 > cache_count[8]=123 > cache_count[24]=122 > > In my opinion, it is not realistic to think that the mempool cache will > remain aligned to cachelines. In these conditions, it looks better to > keep the structure packed to avoid wasting memory.
I agree that is a special use case to only access the mempool cache in bursts of 32 objects, so the accesses are always cache line aligned. (Generalized, the burst size must not be 32; a burst size that is a multiple of RTE_CACHE_LINE_SIZE/sizeof(void*), i.e. a burst size of 8 on a 64-bit architecture, will do.) Adding a hole of 52 byte per mempool cache is nothing, considering that the mempool cache already uses 8 KB (RTE_MEMPOOL_CACHE_MAX_SIZE * 2 * sizeof(void*) = 1024 * 8 byte) for the objects. Also - assuming that memory allocations are cache line aligned - the 52 byte of unused memory cannot be used regardless if they are before or after the objects. Instead of having 52 B unused after the objects, we might as well have a hole of 52 B unused before the objects. In other words: There is really no downside to this. Jerin also presented a separate argument for moving the objects to another cache line than the len field: The risk for "load-after-store stall" when loading the len field after storing objects in cache line0 [1]. [1]: http://inbox.dpdk.org/dev/calbae1p4zfydlwoqukn5q-v-ntvc_ubwmwjhav2uvbxqryt...@mail.gmail.com/ A new idea just popped into my head: The hot debug statistics counters (put_bulk, put_objs, get_success_bulk, get_success_objs) could be moved to this free space, reducing the need to touch another cache line for debug counters. I haven’t thought this idea through yet; it might conflict with Jerin's comment. > > Olivier > > > > > > Credits go to Olivier Matz for the nice ASCII graphics. > > > > Signed-off-by: Morten Brørup <m...@smartsharesystems.com> > > --- > > lib/mempool/rte_mempool.h | 6 ++++-- > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h > > index 1f5707f46a..3725a72951 100644 > > --- a/lib/mempool/rte_mempool.h > > +++ b/lib/mempool/rte_mempool.h > > @@ -86,11 +86,13 @@ struct rte_mempool_cache { > > uint32_t size; /**< Size of the cache */ > > uint32_t flushthresh; /**< Threshold before we flush excess > elements */ > > uint32_t len; /**< Current cache count */ > > - /* > > + /** > > + * Cache objects > > + * > > * Cache is allocated to this size to allow it to overflow in > certain > > * cases to avoid needless emptying of cache. > > */ > > - void *objs[RTE_MEMPOOL_CACHE_MAX_SIZE * 2]; /**< Cache objects */ > > + void *objs[RTE_MEMPOOL_CACHE_MAX_SIZE * 2] __rte_cache_aligned; > > } __rte_cache_aligned; > > > > /** > > -- > > 2.17.1 > >
