Re: [PATCH] mempool: cache align mempool cache objects

[Olivier Matz <olivier.matz@6wind.com>]

On Thu, Oct 27, 2022 at 11:22:07AM +0200, Morten Brørup wrote:
> > From: Olivier Matz [mailto:olivier.matz@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.)

Is there a real situation where it happens to always have read/write
accesses per bulks of 32? From what I see in my quick test, it is not
the case, even with testpmd.

> 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.

Correct, the memory waste argument to nack the patch is invalid.

> 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/

I'll be prudent on this justification without numbers. The case where we
access to the objects of the first cache line (among several KB) is
maybe not that frequent.

> 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.

Yes, but since the stats are only enabled when RTE_LIBRTE_MEMPOOL_DEBUG
is set, it won't have any impact on non-debug builds.


Honnestly, I find it hard to convince myself that it is a real
optimization. I don't see any reason why it would be slower though. So
since we already broke the mempool cache struct ABI in a previous
commit, and since it won't consume more memory, I'm ok to include that
patch. It would be great to have numbers to put some weight in the
balance.




> 
> > 
> > Olivier
> > 
> > 
> > >
> > > Credits go to Olivier Matz for the nice ASCII graphics.
> > >
> > > Signed-off-by: Morten Brørup <mb@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
> > >
>