From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: "Morten Brørup" <mb@smartsharesystems.com>,
"Ola Liljedahl" <Ola.Liljedahl@arm.com>,
"Mattias Rönnblom" <mattias.ronnblom@ericsson.com>,
"dev@dpdk.org" <dev@dpdk.org>
Cc: "thomas@monjalon.net" <thomas@monjalon.net>,
David Marchand <david.marchand@redhat.com>,
"onar.olsen@ericsson.com" <onar.olsen@ericsson.com>,
nd <nd@arm.com>,
"konstantin.ananyev@intel.com" <konstantin.ananyev@intel.com>,
"stephen@networkplumber.org" <stephen@networkplumber.org>,
nd <nd@arm.com>, nd <nd@arm.com>
Subject: RE: [PATCH v3] eal: add seqlock
Date: Sat, 2 Apr 2022 22:01:56 +0000 [thread overview]
Message-ID: <DBAPR08MB58148BBFAA4CFE586D4249B598E39@DBAPR08MB5814.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35D86F9A@smartserver.smartshare.dk>
<snip>
> >
> > > > +__rte_experimental
> > > > +static inline bool
> > > > +rte_seqlock_read_tryunlock(const rte_seqlock_t *seqlock, uint32_t
> > > > +*begin_sn) {
> > > > + uint32_t end_sn;
> > > > +
> > > > + /* make sure the data loads happens before the sn load */
> > > > + rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> > > > +
> > > > + end_sn = __atomic_load_n(&seqlock->sn, __ATOMIC_RELAXED);
> > >
> > > Since we are reading and potentially returning the sequence number
> > here
> > > (repeating the read of the protected data), we need to use load-
> > acquire.
> > > I assume it is not expected that the user will call
> > > rte_seqlock_read_lock() again.
> > Good point, we need a load-acquire (due to changes done in v3).
> >
> > >
> > > Seeing this implementation, I might actually prefer the original
> > > implementation, I think it is cleaner. But I would like for the
> > > begin
> > function
> > > also to wait for an even sequence number, the end function would
> > > only
> > have
> > > to check for same sequence number, this might improve performance a
> > little
> > > bit as readers won't perform one or several broken reads while a
> > write is in
> > > progress. The function names are a different thing though.
> > I think we need to be optimizing for the case where there is no
> > contention between readers and writers (as that happens most of the
> > time). From this perspective, not checking for an even seq number in
> > the begin function would reduce one 'if' statement.
>
> I might be siding with Ola on this, but with a twist: The read_lock() should not
> wait, but test. (Or both variants could be available. Or all three, including the
> variant without checking for an even sequence number.)
>
> My argument for this is: The write operation could take a long time to
> complete, and while this goes on, it is good for the reading threads to know at
> entry of their critical read section that the read operation will fail, so they can
> take the alternative code path instead of proceeding into the critical read
> section. Otherwise, the reading threads have to waste time reading the
> protected data, only to discard them at the end. It's an optimization based on
> the assumption that reading the protected data has some small cost, because
> this small cost adds up if done many times during a longwinded write
> operation.
>
> And, although checking for the sequence number in read_trylock() adds an 'if'
> statement to it, that 'if' statement should be surrounded by likely() to reduce
> its cost in the case we are optimizing for, i.e. when no write operation is
> ongoing.
This 'if' statement can be part of the application code as well. This would allow for multiple models to exist.
>
> This means that read_trylock() returns a boolean, and the sequence number is
> returned in an output parameter.
>
> Please note that it doesn't change the fact that read_tryunlock() can still fail,
> even though read_trylock() gave the go-ahead.
>
> I'm trying to highlight that while we all agree to optimize for the case of
> reading while no writing is ongoing, there might be opportunity for optimizing
> for the opposite case (i.e. trying to read while writing is ongoing) at the same
> time.
>
> I only hope it can be done with negligent performance cost for the primary
> case.
>
> I'll respectfully leave the hardcore implementation details and performance
> considerations to you experts in this area. :-)
next prev parent reply other threads:[~2022-04-02 22:02 UTC|newest]
Thread overview: 104+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-22 16:10 DPDK seqlock Mattias Rönnblom
2022-03-22 16:46 ` Ananyev, Konstantin
2022-03-24 4:52 ` Honnappa Nagarahalli
2022-03-24 5:06 ` Stephen Hemminger
2022-03-24 11:34 ` Mattias Rönnblom
2022-03-25 20:24 ` [RFC] eal: add seqlock Mattias Rönnblom
2022-03-25 21:10 ` Stephen Hemminger
2022-03-26 14:57 ` Mattias Rönnblom
2022-03-27 14:49 ` Ananyev, Konstantin
2022-03-27 17:42 ` Mattias Rönnblom
2022-03-28 10:53 ` Ananyev, Konstantin
2022-03-28 14:06 ` Ola Liljedahl
2022-03-29 8:32 ` Mattias Rönnblom
2022-03-29 13:20 ` Ananyev, Konstantin
2022-03-30 10:07 ` [PATCH] " Mattias Rönnblom
2022-03-30 10:50 ` Morten Brørup
2022-03-30 11:24 ` Tyler Retzlaff
2022-03-30 11:25 ` Mattias Rönnblom
2022-03-30 14:26 ` [PATCH v2] " Mattias Rönnblom
2022-03-31 7:46 ` Mattias Rönnblom
2022-03-31 9:04 ` Ola Liljedahl
2022-03-31 9:25 ` Morten Brørup
2022-03-31 9:38 ` Ola Liljedahl
2022-03-31 10:03 ` Morten Brørup
2022-03-31 11:44 ` Ola Liljedahl
2022-03-31 11:50 ` Morten Brørup
2022-03-31 14:02 ` Mattias Rönnblom
2022-04-01 15:07 ` [PATCH v3] " Mattias Rönnblom
2022-04-02 0:21 ` Honnappa Nagarahalli
2022-04-02 11:01 ` Morten Brørup
2022-04-02 19:38 ` Honnappa Nagarahalli
2022-04-10 13:51 ` [RFC 1/3] eal: add macro to warn for unused function return values Mattias Rönnblom
2022-04-10 13:51 ` [RFC 2/3] eal: emit warning for unused trylock return value Mattias Rönnblom
2022-04-10 13:51 ` [RFC 3/3] examples/bond: fix invalid use of trylock Mattias Rönnblom
2022-04-11 1:01 ` Min Hu (Connor)
2022-04-11 14:32 ` Mattias Rönnblom
2022-04-11 11:25 ` David Marchand
2022-04-11 14:33 ` Mattias Rönnblom
2022-04-10 18:02 ` [RFC 1/3] eal: add macro to warn for unused function return values Stephen Hemminger
2022-04-10 18:50 ` Mattias Rönnblom
2022-04-11 7:17 ` Morten Brørup
2022-04-11 14:29 ` Mattias Rönnblom
2022-04-11 9:16 ` Bruce Richardson
2022-04-11 14:27 ` Mattias Rönnblom
2022-04-11 15:15 ` [PATCH " Mattias Rönnblom
2022-04-11 15:15 ` [PATCH 2/3] eal: emit warning for unused trylock return value Mattias Rönnblom
2022-04-11 15:29 ` Morten Brørup
2022-04-11 15:15 ` [PATCH 3/3] examples/bond: fix invalid use of trylock Mattias Rönnblom
2022-04-14 12:06 ` David Marchand
2022-04-11 15:25 ` [PATCH 1/3] eal: add macro to warn for unused function return values Morten Brørup
2022-04-11 18:24 ` [RFC " Tyler Retzlaff
2022-04-03 6:10 ` [PATCH v3] eal: add seqlock Mattias Rönnblom
2022-04-03 17:27 ` Honnappa Nagarahalli
2022-04-03 18:37 ` Ola Liljedahl
2022-04-04 21:56 ` Honnappa Nagarahalli
2022-04-03 6:33 ` Mattias Rönnblom
2022-04-03 17:37 ` Honnappa Nagarahalli
2022-04-08 13:45 ` Mattias Rönnblom
2022-04-02 18:15 ` Ola Liljedahl
2022-04-02 19:31 ` Honnappa Nagarahalli
2022-04-02 20:36 ` Morten Brørup
2022-04-02 22:01 ` Honnappa Nagarahalli [this message]
2022-04-03 18:11 ` Ola Liljedahl
2022-04-03 6:51 ` Mattias Rönnblom
2022-03-31 13:51 ` [PATCH v2] " Mattias Rönnblom
2022-04-02 0:54 ` Stephen Hemminger
2022-04-02 10:25 ` Morten Brørup
2022-04-02 17:43 ` Ola Liljedahl
2022-03-31 13:38 ` Mattias Rönnblom
2022-03-31 14:53 ` Ola Liljedahl
2022-04-02 0:52 ` Stephen Hemminger
2022-04-03 6:23 ` Mattias Rönnblom
2022-04-02 0:50 ` Stephen Hemminger
2022-04-02 17:54 ` Ola Liljedahl
2022-04-02 19:37 ` Honnappa Nagarahalli
2022-04-05 20:16 ` Stephen Hemminger
2022-04-08 13:50 ` Mattias Rönnblom
2022-04-08 14:24 ` [PATCH v4] " Mattias Rönnblom
2022-04-08 15:17 ` Stephen Hemminger
2022-04-08 16:24 ` Mattias Rönnblom
2022-04-08 15:19 ` Stephen Hemminger
2022-04-08 16:37 ` Mattias Rönnblom
2022-04-08 16:48 ` Mattias Rönnblom
2022-04-12 17:27 ` Ananyev, Konstantin
2022-04-28 10:28 ` David Marchand
2022-05-01 13:46 ` Mattias Rönnblom
2022-05-01 14:03 ` [PATCH v5] " Mattias Rönnblom
2022-05-01 14:22 ` Mattias Rönnblom
2022-05-02 6:47 ` David Marchand
2022-05-01 20:17 ` Stephen Hemminger
2022-05-02 4:51 ` Mattias Rönnblom
2022-05-06 1:26 ` fengchengwen
2022-05-06 1:33 ` Honnappa Nagarahalli
2022-05-06 4:17 ` fengchengwen
2022-05-06 5:19 ` Honnappa Nagarahalli
2022-05-06 7:03 ` fengchengwen
2022-05-08 11:56 ` Mattias Rönnblom
2022-05-08 12:12 ` [PATCH v6] " Mattias Rönnblom
2022-05-08 16:10 ` Stephen Hemminger
2022-05-08 19:40 ` Mattias Rönnblom
2022-05-09 3:48 ` Stephen Hemminger
2022-05-09 6:26 ` Morten Brørup
2022-05-13 6:27 ` Mattias Rönnblom
2022-03-23 12:04 ` DPDK seqlock Morten Brørup
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