Re: 8.3 / 8.2.6 restore comparison

Heikki, are there any TODO items here?

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Heikki Linnakangas wrote:
> Joshua D. Drake wrote:
> > On Sun, 24 Feb 2008 00:43:18 +0000
> > "Heikki Linnakangas" <heikki@enterprisedb.com> wrote:
> >  
> >> Incidentally, I've been working on a patch to speed up CopyReadLine.
> >> I was going to run some more tests first, but since we're talking
> >> about it, I guess I should just post the patch. I'll post to
> >> pgsql-patches shortly.
> > 
> > On your post to patches you mentioned only about a 5% improvement.
> > Don't get me wrong, 5% is 5% and I respect it greatly but as far as I
> > can tell we are about 300% behind the curve.
> 
> Yeah. Looking at the profile, the time is spent really all over the 
> place. There's no one clear bottleneck to focus on. I think we could do 
> a few more ~5% improvements, but
> 
> At some point, I think we have to bite the bullet and find a way to use 
> multiple CPUs for a single load. I don't have any good ideas or plans 
> for that, but hopefully someone does.
> 
> > My tests were maxing out at ~22G an hour. On hardware that can do 
> > in 360G an hour and that is assuming > 50% overhead between OS, libs,
> > etc... I have no choice but to conclude we have a much, much deeper and
> > fundamental issue going on with COPY. I am inspired by Itagaki Takahiro
> > and his batch update of indexes which should help greatly overall but
> > doesn't help my specific issue.
> 
> Yep, the index build idea is an I/O improvement, not a CPU one.
> 
> > Forgive me for not being a C programmer and Alvaro is not online so I
> > would vet these questions with him first.
> > 
> > I know that copy is in theory a bulk loader but, when performing the
> > readline how many lines are we reading?  Do we read up to 8192? Or do we
> > shove in say 8megs of data before we invoke DoCopy?
> 
> We read 64 KB at a time, and then CopyReadLineText returns one line at a 
> time from that buffer.
> 
> Looking at your profile more, and after the memchr patch, the "raw input 
> side" of copy, consisting of reading the data from disk in 64KB blocks, 
> splitting that into lines, and splitting lines into columns, still takes 
> ~20% of the CPU time. I suspect CopyReadAttributesText is the biggest 
> culprit there.
> 
> You could avoid the ~8% spent in XLogInsert in PostgreSQL 8.3, by 
> creating the table (or truncating it) in the same transaction with the COPY.
> 
> After that, heap_formtuple is high on the list. I wonder if we could do 
> something about that.
> 
> > I am just curious if there is some simple low hanging fruit that is
> > possibly missing.
> 
> I don't see any piece of code that's causing problems. We can shave off 
> a few percents here and there I think, but don't expect a 300% 
> improvement anytime soon. A few ideas I've thought about are:
> 
> - use a specialized version of strtol, for base 10. That won't help on 
> your table, but I've seen strtol consume a significant amount of time on 
> tables with numeric/integer columns.
> 
> - Instead of pallocing and memcpying the text fields, leave a little bit 
> of room between fields in the attribute_buf, and write the varlen header 
> there directly. This might help you since your table has a lot of text 
> fields.
> 
> - Instead of the normal PG function calling conventions, provide 
> specialized fastpath input functions for the most common data types. 
> InputFunctionCall consumed 4.5% of the CPU time in your profile.
> 
> - Use a simpler memory context implementation, that's like a stack with 
> no pfree support, for the per-tuple context.
> 
> -- 
>    Heikki Linnakangas
>    EnterpriseDB   http://www.enterprisedb.com
> 
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