[PATCH] mempool: cache align mempool cache objects

[Morten Brørup]

> From: Olivier Matz [mailto:olivier.matz at 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_UBWmWjhaV2uVBXQRytSSA@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 <mb at 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
> >