From: Stephen Hemminger <stephen@networkplumber.org>
To: "Mattias Rönnblom" <hofors@lysator.liu.se>
Cc: "Morten Brørup" <mb@smartsharesystems.com>,
"Tyler Retzlaff" <roretzla@linux.microsoft.com>,
"Thomas Monjalon" <thomas@monjalon.net>,
"Bruce Richardson" <bruce.richardson@intel.com>,
dev@dpdk.org
Subject: Re: [PATCH 1/4] latencystats: use alloca instead of vla trivial
Date: Sun, 7 Apr 2024 10:00:08 -0700 [thread overview]
Message-ID: <20240407100008.361cb45a@hermes.local> (raw)
In-Reply-To: <0c4ba365-94db-46ed-b843-2a70b0948368@lysator.liu.se>
On Sun, 7 Apr 2024 11:36:59 +0200
Mattias Rönnblom <hofors@lysator.liu.se> wrote:
> On 2024-04-06 17:28, Morten Brørup wrote:
> >> From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com]
> >> Sent: Thursday, 4 April 2024 19.15
> >>
> >> RFC sample illustrating simple conversion of VLA to alloca().
> >>
> >> Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
> >> ---
> >
> > [...]
> >
> >> --- a/lib/latencystats/rte_latencystats.c
> >> +++ b/lib/latencystats/rte_latencystats.c
> >> @@ -159,7 +159,7 @@ struct latency_stats_nameoff {
> >> {
> >> unsigned int i, cnt = 0;
> >> uint64_t now;
> >> - float latency[nb_pkts];
> >> + float *latency = alloca(sizeof(float) * nb_pkts);
> >
> > In cases where we are processing packet bursts, I would prefer introducing a global #define RTE_MAX_PKT_BURST_SIZE, indicating the max packet burst size supported by libraries and drivers.
>
> First question: what is meant by a "packet" here? An mbuf? A
> network-layer PDU? Something that in some way relates to zero or more
> packets, like an rte_event? Or just any object that are sent or receive
> of some DPDK API in batches or bursts?
>
> Second question: is RTE_MAX_PKT_BURST_SIZE meant as an upper bound, so
> no API can consumer or produce a burst larger than this, it does all
> APIs literally have to support that burst size.
>
> Third question: why not just keep VLAs?
>
> > For reference, rte_config.h already has #define RTE_GRAPH_BURST_SIZE 256.
> >
> > Such a common define should also be used by functions such as rte_pktmbuf_free_bulk(); although it also supports segmented packets, so it must still be able to handle more mbufs.
> > https://elixir.bootlin.com/dpdk/v24.03/source/lib/mbuf/rte_mbuf.c#L486
> >
Looking at the maths here, calc_lantency can be seriously improved:
- the calc latency is in the fast path. for transmit.
- it is doing floating point math; floating point is much slower than doing
fixed point
- the latency[] array is a temporary, it should be possible to compute
total latency without it.
- it acquires a lock, in order to achieve DPDK level performance of 40 Mpps, it is
necessary to not do absolute minimum of locking.
next prev parent reply other threads:[~2024-04-07 17:00 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-07 19:32 RFC acceptable handling of VLAs across toolchains Tyler Retzlaff
2023-11-08 2:31 ` Stephen Hemminger
2023-11-08 3:25 ` Tyler Retzlaff
2023-11-08 8:19 ` Morten Brørup
2023-11-08 16:51 ` Stephen Hemminger
2023-11-08 17:48 ` Morten Brørup
2023-11-09 10:25 ` RFC: default burst sizes in rte_config Morten Brørup
2023-11-09 20:26 ` RFC acceptable handling of VLAs across toolchains Tyler Retzlaff
2024-03-21 0:12 ` Tyler Retzlaff
2024-04-04 17:15 ` [PATCH 0/4] RFC samples converting VLA to alloca Tyler Retzlaff
2024-04-04 17:15 ` [PATCH 1/4] latencystats: use alloca instead of vla trivial Tyler Retzlaff
2024-04-06 15:28 ` Morten Brørup
2024-04-07 9:36 ` Mattias Rönnblom
2024-04-07 17:00 ` Stephen Hemminger [this message]
2024-04-04 17:15 ` [PATCH 2/4] hash: " Tyler Retzlaff
2024-04-06 16:01 ` Morten Brørup
2024-04-04 17:15 ` [PATCH 3/4] vhost: use alloca instead of vla sizeof Tyler Retzlaff
2024-04-06 22:30 ` Morten Brørup
2024-04-04 17:15 ` [PATCH 4/4] dispatcher: use alloca instead of vla multi dimensional Tyler Retzlaff
2024-04-06 15:49 ` Morten Brørup
2024-04-07 9:31 ` [PATCH 0/4] RFC samples converting VLA to alloca Mattias Rönnblom
2024-04-07 11:07 ` Morten Brørup
2024-04-07 17:03 ` Stephen Hemminger
2024-04-08 15:27 ` Tyler Retzlaff
2024-04-08 15:53 ` Morten Brørup
2024-04-09 8:28 ` Konstantin Ananyev
2024-04-09 15:08 ` Tyler Retzlaff
2024-04-10 9:58 ` Konstantin Ananyev
2024-04-10 17:03 ` Tyler Retzlaff
2024-04-10 7:32 ` Mattias Rönnblom
2024-04-10 7:52 ` Morten Brørup
2024-04-10 17:04 ` Tyler Retzlaff
2024-04-10 7:27 ` Mattias Rönnblom
2024-04-10 17:10 ` Tyler Retzlaff
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