Re: Parallel copy - Mailing list pgsql-hackers

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.

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