From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Olivier Matz" <olivier.matz@6wind.com>
Cc: <andrew.rybchenko@oktetlabs.ru>, <jerinj@marvell.com>,
<thomas@monjalon.net>, <bruce.richardson@intel.com>,
<dev@dpdk.org>
Subject: RE: [PATCH] mempool: cache align mempool cache objects
Date: Thu, 27 Oct 2022 11:22:07 +0200 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D87450@smartserver.smartshare.dk> (raw)
In-Reply-To: <Y1pCovW8C2TbCq42@platinum>
> 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.)
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@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
> >
next prev parent reply other threads:[~2022-10-27 9:22 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-26 15:34 [RFC] mempool: rte_mempool_do_generic_get optimizations Morten Brørup
2022-01-06 12:23 ` [PATCH] mempool: optimize incomplete cache handling Morten Brørup
2022-01-06 16:55 ` Jerin Jacob
2022-01-07 8:46 ` Morten Brørup
2022-01-10 7:26 ` Jerin Jacob
2022-01-10 10:55 ` Morten Brørup
2022-01-14 16:36 ` [PATCH] mempool: fix get objects from mempool with cache Morten Brørup
2022-01-17 17:35 ` Bruce Richardson
2022-01-18 8:25 ` Morten Brørup
2022-01-18 9:07 ` Bruce Richardson
2022-01-24 15:38 ` Olivier Matz
2022-01-24 16:11 ` Olivier Matz
2022-01-28 10:22 ` Morten Brørup
2022-01-17 11:52 ` [PATCH] mempool: optimize put objects to " Morten Brørup
2022-01-19 14:52 ` [PATCH v2] mempool: fix " Morten Brørup
2022-01-19 15:03 ` [PATCH v3] " Morten Brørup
2022-01-24 15:39 ` Olivier Matz
2022-01-28 9:37 ` Morten Brørup
2022-02-02 8:14 ` [PATCH v2] mempool: fix get objects from " Morten Brørup
2022-06-15 21:18 ` Morten Brørup
2022-09-29 10:52 ` Morten Brørup
2022-10-04 12:57 ` Andrew Rybchenko
2022-10-04 15:13 ` Morten Brørup
2022-10-04 15:58 ` Andrew Rybchenko
2022-10-04 18:09 ` Morten Brørup
2022-10-06 13:43 ` Aaron Conole
2022-10-04 16:03 ` Morten Brørup
2022-10-04 16:36 ` Morten Brørup
2022-10-04 16:39 ` Morten Brørup
2022-02-02 10:33 ` [PATCH v4] mempool: fix mempool cache flushing algorithm Morten Brørup
2022-04-07 9:04 ` Morten Brørup
2022-04-07 9:14 ` Bruce Richardson
2022-04-07 9:26 ` Morten Brørup
2022-04-07 10:32 ` Bruce Richardson
2022-04-07 10:43 ` Bruce Richardson
2022-04-07 11:36 ` Morten Brørup
2022-10-04 20:01 ` Morten Brørup
2022-10-09 11:11 ` [PATCH 1/2] mempool: check driver enqueue result in one place Andrew Rybchenko
2022-10-09 11:11 ` [PATCH 2/2] mempool: avoid usage of term ring on put Andrew Rybchenko
2022-10-09 13:08 ` Morten Brørup
2022-10-09 13:14 ` Andrew Rybchenko
2022-10-09 13:01 ` [PATCH 1/2] mempool: check driver enqueue result in one place Morten Brørup
2022-10-09 13:19 ` [PATCH v4] mempool: fix mempool cache flushing algorithm Andrew Rybchenko
2022-10-04 12:53 ` [PATCH v3] mempool: fix get objects from mempool with cache Andrew Rybchenko
2022-10-04 14:42 ` Morten Brørup
2022-10-07 10:44 ` [PATCH v4] " Andrew Rybchenko
2022-10-08 20:56 ` Thomas Monjalon
2022-10-11 20:30 ` Copy-pasted code should be updated Morten Brørup
2022-10-11 21:47 ` Honnappa Nagarahalli
2022-10-30 8:44 ` Morten Brørup
2022-10-30 22:50 ` Honnappa Nagarahalli
2022-10-14 14:01 ` [PATCH v4] mempool: fix get objects from mempool with cache Olivier Matz
2022-10-09 13:37 ` [PATCH v6 0/4] mempool: fix mempool cache flushing algorithm Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 1/4] mempool: check driver enqueue result in one place Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 2/4] mempool: avoid usage of term ring on put Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 3/4] mempool: fix cache flushing algorithm Andrew Rybchenko
2022-10-09 14:31 ` Morten Brørup
2022-10-09 14:51 ` Andrew Rybchenko
2022-10-09 15:08 ` Morten Brørup
2022-10-14 14:01 ` Olivier Matz
2022-10-14 15:57 ` Morten Brørup
2022-10-14 19:50 ` Olivier Matz
2022-10-15 6:57 ` Morten Brørup
2022-10-18 16:32 ` Jerin Jacob
2022-10-09 13:37 ` [PATCH v6 4/4] mempool: flush cache completely on overflow Andrew Rybchenko
2022-10-09 14:44 ` Morten Brørup
2022-10-14 14:01 ` Olivier Matz
2022-10-10 15:21 ` [PATCH v6 0/4] mempool: fix mempool cache flushing algorithm Thomas Monjalon
2022-10-11 19:26 ` Morten Brørup
2022-10-26 14:09 ` Thomas Monjalon
2022-10-26 14:26 ` Morten Brørup
2022-10-26 14:44 ` [PATCH] mempool: cache align mempool cache objects Morten Brørup
2022-10-26 19:44 ` Andrew Rybchenko
2022-10-27 8:34 ` Olivier Matz
2022-10-27 9:22 ` Morten Brørup [this message]
2022-10-27 11:42 ` Olivier Matz
2022-10-27 12:11 ` Morten Brørup
2022-10-27 15:20 ` Olivier Matz
2022-10-28 6:35 ` [PATCH v3 1/2] " Morten Brørup
2022-10-28 6:35 ` [PATCH v3 2/2] mempool: optimized debug statistics Morten Brørup
2022-10-28 6:41 ` [PATCH v4 1/2] mempool: cache align mempool cache objects Morten Brørup
2022-10-28 6:41 ` [PATCH v4 2/2] mempool: optimized debug statistics Morten Brørup
2022-10-30 9:09 ` Morten Brørup
2022-10-30 9:16 ` Thomas Monjalon
2022-10-30 9:17 ` [PATCH v4 1/2] mempool: cache align mempool cache objects Thomas Monjalon
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