Re: Parallel copy

On 30/10/2020 18:36, Heikki Linnakangas wrote:
> I find this design to be very complicated. Why does the line-boundary
> information need to be in shared memory? I think this would be much
> simpler if each worker grabbed a fixed-size block of raw data, and
> processed that.
>
> In your patch, the leader process scans the input to find out where one
> line ends and another begins, and because of that decision, the leader
> needs to make the line boundaries available in shared memory, for the
> worker processes. If we moved that responsibility to the worker
> processes, you wouldn't need to keep the line boundaries in shared
> memory. A worker would only need to pass enough state to the next worker
> to tell it where to start scanning the next block.

Here's a high-level sketch of how I'm imagining this to work:

The shared memory structure consists of a queue of blocks, arranged as a
ring buffer. Each block is of fixed size, and contains 64 kB of data,
and a few fields for coordination:

typedef struct
{
/* Current state of the block */
pg_atomic_uint32 state;

/* starting offset of first line within the block */
int startpos;

char data[64 kB];
} ParallelCopyDataBlock;

Where state is one of:

enum {
FREE, /* buffer is empty */
FILLED, /* leader has filled the buffer with raw data */
READY, /* start pos has been filled in, but no worker process
has claimed the block yet */
PROCESSING, /* worker has claimed the block, and is processing it */
}

State changes FREE -> FILLED -> READY -> PROCESSING -> FREE. As the COPY
progresses, the ring of blocks will always look something like this:

blk 0 startpos 0: PROCESSING [worker 1]
blk 1 startpos 12: PROCESSING [worker 2]
blk 2 startpos 10: READY
blk 3 starptos -: FILLED
blk 4 startpos -: FILLED
blk 5 starptos -: FILLED
blk 6 startpos -: FREE
blk 7 startpos -: FREE

Typically, each worker process is busy processing a block. After the
blocks being processed, there is one block in READY state, and after
that, blocks in FILLED state.

Leader process:

The leader process is simple. It picks the next FREE buffer, fills it
with raw data from the file, and marks it as FILLED. If no buffers are
FREE, wait.

Worker process:

1. Claim next READY block from queue, by changing its state to
PROCESSING. If the next block is not READY yet, wait until it is.

2. Start scanning the block from 'startpos', finding end-of-line
markers. (in CSV mode, need to track when we're in-quotes).

3. When you reach the end of the block, if the last line continues to
next block, wait for the next block to become FILLED. Peek into the
next block, and copy the remaining part of the split line to a local
buffer, and set the 'startpos' on the next block to point to the end
of the split line. Mark the next block as READY.

4. Process all the lines in the block, call input functions, insert
rows.

5. Mark the block as DONE.

In this design, you don't need to keep line boundaries in shared memory,
because each worker process is responsible for finding the line
boundaries of its own block.

There's a point of serialization here, in that the next block cannot be
processed, until the worker working on the previous block has finished
scanning the EOLs, and set the starting position on the next block,
putting it in READY state. That's not very different from your patch,
where you had a similar point of serialization because the leader
scanned the EOLs, but I think the coordination between processes is
simpler here.

- Heikki