From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Cc: tml <tarantool-patches@dev.tarantool.org>
Subject: Re: [Tarantool-patches] [PATCH 1/5] qsync: eliminate redundant writes
Date: Sat, 11 Jul 2020 00:04:39 +0300 [thread overview]
Message-ID: <20200710210439.GG1999@grain> (raw)
In-Reply-To: <bbbafe41-1773-2b80-6ac7-1661711cde9b@tarantool.org>
On Fri, Jul 10, 2020 at 10:31:22PM +0200, Vladislav Shpilevoy wrote:
> Hi!
>
> On 10/07/2020 09:56, Cyrill Gorcunov wrote:
> > Instead of updating is_sync variable on every
> > cycle write it once.
> >
> > Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
> > ---
> > src/box/txn.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/box/txn.c b/src/box/txn.c
> > index a2df23833..49b2b2649 100644
> > --- a/src/box/txn.c
> > +++ b/src/box/txn.c
> > @@ -567,8 +567,12 @@ txn_journal_entry_new(struct txn *txn)
> > if (stmt->row == NULL)
> > continue;
> >
> > - is_sync = is_sync || (stmt->space != NULL &&
> > - stmt->space->def->opts.is_sync);
> > + if (!is_sync) {
> > + if (stmt->space != NULL &&
> > + stmt->space->def->opts.is_sync) {
> > + is_sync = true;
> > + }
> > + }
> >
> > if (stmt->row->replica_id == 0)
> > *local_row++ = stmt->row;
>
> The 'redundant' writes were done intentionally, to avoid unnecessary
> branching. I see no point in this change.
Vlad, I'm sorry to say that your assumption about unnecessary branching
is simply wrong. Look, the || operator means that there is a read and
compare on asm level, iow there is _already_ a branch present while it
might be not that obvious on C level.
0000000000002578 <txn_journal_entry_new>:
...
26ab: 84 c0 test %al,%al
26ad: 74 07 je 26b6 <txn_journal_entry_new+0x13e>
26af: b8 01 00 00 00 mov $0x1,%eax
26b4: eb 05 jmp 26bb <txn_journal_entry_new+0x143>
26b6: b8 00 00 00 00 mov $0x0,%eax
26bb: 88 45 e7 mov %al,-0x19(%rbp)
26be: 80 65 e7 01 andb $0x1,-0x19(%rbp)
see this test+je pair? IOW, in my version we eliminate redundant write operation.
And just for the future -- branching is cheaper than write operation on hot paths.
next prev parent reply other threads:[~2020-07-10 21:04 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-10 7:56 [Tarantool-patches] [PATCH 0/5] qsync: code cleanup Cyrill Gorcunov
2020-07-10 7:56 ` [Tarantool-patches] [PATCH 1/5] qsync: eliminate redundant writes Cyrill Gorcunov
2020-07-10 20:31 ` Vladislav Shpilevoy
2020-07-10 21:04 ` Cyrill Gorcunov [this message]
2020-07-10 7:56 ` [Tarantool-patches] [PATCH 2/5] qsync: add a comment about sync txn in journal allocation Cyrill Gorcunov
2020-07-10 20:33 ` Vladislav Shpilevoy
2020-07-10 20:34 ` Vladislav Shpilevoy
2020-07-10 21:07 ` Cyrill Gorcunov
2020-07-10 21:08 ` Vladislav Shpilevoy
2020-07-11 16:08 ` Vladislav Shpilevoy
2020-07-10 7:56 ` [Tarantool-patches] [PATCH 3/5] qsync: txn_commit_async -- drop redundant variable Cyrill Gorcunov
2020-07-10 20:35 ` Vladislav Shpilevoy
2020-07-10 21:10 ` Cyrill Gorcunov
2020-07-10 21:28 ` Vladislav Shpilevoy
2020-07-10 21:36 ` Cyrill Gorcunov
2020-07-11 14:10 ` Vladislav Shpilevoy
2020-07-11 15:18 ` Cyrill Gorcunov
2020-07-10 7:56 ` [Tarantool-patches] [PATCH 4/5] qsync: txn_commit -- use txn flag instead of caching variable Cyrill Gorcunov
2020-07-10 20:36 ` Vladislav Shpilevoy
2020-07-10 21:27 ` Cyrill Gorcunov
2020-07-10 7:56 ` [Tarantool-patches] [PATCH 5/5] qsync: sanitize txn_limbo_on_rollback Cyrill Gorcunov
2020-07-10 20:38 ` Vladislav Shpilevoy
2020-07-11 15:46 ` Cyrill Gorcunov
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