[Tarantool-patches] [PATCH 1/5] qsync: eliminate redundant writes
Cyrill Gorcunov
gorcunov at gmail.com
Sat Jul 11 00:04:39 MSK 2020
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 at 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.
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