Re: Parallel copy - Mailing list pgsql-hackers

On Sat, Nov 7, 2020 at 7:01 PM vignesh C <vignesh21@gmail.com> wrote:
>
> On Thu, Nov 5, 2020 at 6:33 PM Hou, Zhijie <houzj.fnst@cn.fujitsu.com> wrote:
> >
> > Hi
> >
> > >
> > > my $bytes = $ARGV[0];
> > > for(my $i = 0; $i < $bytes; $i+=8){
> > >      print "longdata";
> > > }
> > > print "\n";
> > > --------
> > >
> > > postgres=# copy longdata from program 'perl /tmp/longdata.pl 100000000'
> > > with (parallel 2);
> > >
> > > This gets stuck forever (or at least I didn't have the patience to wait
> > > it finish). Both worker processes are consuming 100% of CPU.
> >
> > I had a look over this problem.
> >
> > the ParallelCopyDataBlock has size limit:
> >         uint8           skip_bytes;
> >         char            data[DATA_BLOCK_SIZE];  /* data read from file */
> >
> > It seems the input line is so long that the leader process run out of the Shared memory among parallel copy workers.
> > And the leader process keep waiting free block.
> >
> > For the worker process, it wait util line_state becomes LINE_LEADER_POPULATED,
> > But leader process won't set the line_state unless it read the whole line.
> >
> > So it stuck forever.
> > May be we should reconsider about this situation.
> >
> > The stack is as follows:
> >
> > Leader stack:
> > #3  0x000000000075f7a1 in WaitLatch (latch=<optimized out>, wakeEvents=wakeEvents@entry=41, timeout=timeout@entry=1, wait_event_info=wait_event_info@entry=150994945) at latch.c:411
> > #4  0x00000000005a9245 in WaitGetFreeCopyBlock (pcshared_info=pcshared_info@entry=0x7f26d2ed3580) at copyparallel.c:1546
> > #5  0x00000000005a98ce in SetRawBufForLoad (cstate=cstate@entry=0x2978a88, line_size=67108864, copy_buf_len=copy_buf_len@entry=65536, raw_buf_ptr=raw_buf_ptr@entry=65536,
> >     copy_raw_buf=copy_raw_buf@entry=0x7fff4cdc0e18) at copyparallel.c:1572
> > #6  0x00000000005a1963 in CopyReadLineText (cstate=cstate@entry=0x2978a88) at copy.c:4058
> > #7  0x00000000005a4e76 in CopyReadLine (cstate=cstate@entry=0x2978a88) at copy.c:3863
> >
> > Worker stack:
> > #0  GetLinePosition (cstate=cstate@entry=0x29e1f28) at copyparallel.c:1474
> > #1  0x00000000005a8aa4 in CacheLineInfo (cstate=cstate@entry=0x29e1f28, buff_count=buff_count@entry=0) at copyparallel.c:711
> > #2  0x00000000005a8e46 in GetWorkerLine (cstate=cstate@entry=0x29e1f28) at copyparallel.c:885
> > #3  0x00000000005a4f2e in NextCopyFromRawFields (cstate=cstate@entry=0x29e1f28, fields=fields@entry=0x7fff4cdc0b48, nfields=nfields@entry=0x7fff4cdc0b44) at copy.c:3615
> > #4  0x00000000005a50af in NextCopyFrom (cstate=cstate@entry=0x29e1f28, econtext=econtext@entry=0x2a358d8, values=0x2a42068, nulls=0x2a42070) at copy.c:3696
> > #5  0x00000000005a5b90 in CopyFrom (cstate=cstate@entry=0x29e1f28) at copy.c:2985
> >
>
> Thanks for providing your thoughts. I have analyzed this issue and I'm
> working on the fix for this, I will be posting a patch for this
> shortly.
>

I have fixed and provided a patch for this at [1]
[1] https://www.postgresql.org/message-id/CALDaNm05FnA-ePvYV_t2%2BWE_tXJymbfPwnm%2Bkc9y1iMkR%2BNbUg%40mail.gmail.com


Regards,
Vignesh
EnterpriseDB: http://www.enterprisedb.com