Parallelize query execution

[brancz]

Currently, arcticDB's query execution is not parallelized, but we want to do that. There are well-known techniques such as vulcano to generalize the parallelization of steps within a query. I think vulcano is a promising direction for arcticDB but I'd be happy for us to explore other possibilities as well.

[albertlockett]

Hey @brancz I agree volcano seems like a good fit.

To add parallelization to articdb, I think we could add an implementation of the exchange operator described in the paper. We could have optimizers in the logicalplan to inject exchange operators in the appropriate places in the query tree.

I took a stab at implementing this. #90

The ExchangeOperator in this cases uses channels instead of IPC like in the paper.. The chan length is used to provide the backpressure that (whereas in the paper they describe using semaphores), and it spawns a number goroutines that are listening to provide the horizontal parallelism (instead of using multiple queues as the paper describes).

Does that sound like it's the right approach? Is it too simplistic?

Of course this code in the PR is still WIP, but it gives us something to iterate on at least. Looking forward to your feedback!

[yeya24]

Compared to the Volcano's 1-tuple-a-time Iterator model, vectorwise batching iterator model used by MonetDB seems better for columnar storage https://www.cidrdb.org/cidr2005/papers/P19.pdf. This is used by TiDB and CRDB as well.

But since we are using Arrow underlying already maybe Volcano is good enough.

[brancz]

I wasn't aware of this paper, thanks for sharing @yeya24! I've only cross-read the paper, but from what I can tell what @albertlockett proposed so far is actually in line with what's described in this paper, as it handles full arrow.Record frames, as opposed to 1-tuple-a-time like you said.

About what Albert wrote:

Does that sound like it's the right approach? Is it too simplistic?

I think what you proposed is very elegant and should work quite well. Only practice will truly tell, but I think the direction looks promising!

[albertlockett]

Thanks @brancz, I'll move forward with this. I'll get the code cleaned up, add tests & error handling and do some profiling to verify if this approach improves performance and what level of parallelism makes sense.

[albertlockett]

@brancz @metalmatze we've managed to get a good performance boost from this parallelism. I wrote a benchmark program (see below) and found that this is about 50% faster with the new code path.

I found that the code in table iterator that was a bit of a bottleneck, so by parallelizing that we're able to take advantage of parallelism in the execution of the physical plan.
https://github.com/polarsignals/arcticdb/pull/90/files#diff-134c48f173480a9772719053c1b3e967756a71c75f84ed4f94f12c71cd91ed30R319

I also tested varying the level of parallelism. Adding more horizontal parallelism gets us improved query performance to a point. I found performance improvements seemed to taper off when I got to a number of workers that was approximately the number of CPU cores on my PC -1 (which is probably expected in a highly cpu bound workload like this benchmark)

How does this all sound to y'all? Does that sound like a good performance improvement? Would we expect more improvment from this technique?

This code in PR is still WIP (I need to fix/write some unit tests)

The raw data and the detailed analysis results can be found here (for reference)
https://docs.google.com/spreadsheets/d/1782MYsiM3rHqb3VK77EJ9VpLxM3ET-WcxROIFQ686Vs/edit#gid=0
These numbers are from running this test on my macbook, which as 12 cpu cores.

Here's the benchmarking program I used (also for reference)

package main

import (
	"context"
	"fmt"
	"log"
	"os"
	"runtime"
	"runtime/pprof"
	"time"

	"github.com/apache/arrow/go/v8/arrow"

	"github.com/apache/arrow/go/v8/arrow/memory"
	log2 "github.com/go-kit/log"
	"github.com/go-kit/log/level"
	"github.com/segmentio/parquet-go"

	"github.com/polarsignals/arcticdb"
	"github.com/polarsignals/arcticdb/dynparquet"
	"github.com/polarsignals/arcticdb/query"
	"github.com/polarsignals/arcticdb/query/logicalplan"
)

func main() {
	var err error
	columnstore := arcticdb.New(nil, 10, 512*1024*1024).WithIndexDegree(3)
	db, err := columnstore.DB("test_db")
	if err != nil {
		panic(err)
	}

	logger := log2.NewLogfmtLogger(log2.NewSyncWriter(os.Stderr))
	logger = level.NewFilter(logger, level.AllowDebug())

	schema := dynparquet.NewSchema("test_schema",
		[]dynparquet.ColumnDefinition{
			{
				Name:          "Column1",
				StorageLayout: parquet.Encoded(parquet.String(), &parquet.RLEDictionary),
				Dynamic:       false,
			},
			{
				Name:          "Column2",
				StorageLayout: parquet.Int(64),
				Dynamic:       false,
			},
		},
		[]dynparquet.SortingColumn{
			dynparquet.Ascending("Column1"),
		})
	tableConfig := arcticdb.NewTableConfig(schema)

	table, err := db.Table("test_table", tableConfig, logger)
	if err != nil {
		panic(err)
	}

	buffer, err := schema.NewBuffer(map[string][]string{})

	log.Printf("num cores = %d\n", runtime.NumCPU())

	log.Println("begin writing table")
	for i := 0; i < 250_000; i++ {
		row := make([]parquet.Value, 0)
		message := fmt.Sprintf("hello%d", i)
		row = append(row, parquet.ValueOf(message).Level(0, 0, 0))
		row = append(row, parquet.ValueOf(10).Level(0, 0, 1))
		_, err = buffer.WriteRows([]parquet.Row{row})
		if err != nil {
			panic(err)
		}

		if i > 0 && i%1000 == 0 {
			_, err = table.InsertBuffer(context.Background(), buffer)
			if err != nil {
				panic(err)
			}
			buffer, err = schema.NewBuffer(map[string][]string{})
			if err != nil {
				panic(err)
			}
		}
	}

	_, err = table.InsertBuffer(context.Background(), buffer)
	if err != nil {
		panic(err)
	}
	table.Sync()

	log.Println("done writing table")
	queryEngine := query.NewEngine(memory.DefaultAllocator, db.TableProvider())
	f, perr := os.Create("cpu.pprof")
	if perr != nil {
		log.Fatal(perr)
	}
	err = pprof.StartCPUProfile(f)
	if err != nil {
		panic(err)
	}

	for j := 0; j < 100; j++ {
		for i := 1; i < 20; i++ {
			os.Setenv("parallelism", fmt.Sprintf("%d", i)) // this is what I was using to vary the parallelism
			query := queryEngine.ScanTable("test_table").
				Filter(
					&logicalplan.BinaryExpr{
						Left:  logicalplan.Col("Column1"),
						Op:    logicalplan.RegExpOp,
						Right: logicalplan.Literal("hello.*"),
					},
				)
			start := time.Now().UnixMicro()
			err = query.Execute(context.Background(), func(ar arrow.Record) error {
				log.Printf("%v", ar)
				return nil
			})
			end := time.Now().UnixMicro()
			log.Printf("parallel = %d | trial = %d | time = %d us\n", i, j, end-start)
		}
	}
	pprof.StopCPUProfile()

	if err != nil {
		msg := err.Error()
		log.Printf("%s", msg)
		log.Printf("%v", err)
	}
}

[albertlockett]

here is a document with some more of my notes
https://docs.google.com/document/d/1-9jX8XafLP3m5hLShSRh1hoMAEKrQTzqS0og2JCOlXg/edit#heading=h.vf1o5lbbldn6

[metalmatze]

That all sounds great and makes sense with the NumCPUs. I would have also been fine with just leaving it at that number and not subtracting 1, but since you've already figured out we can keep NumCPUs - 1.

There's one comment on that PR about errgroups and another nitpick. Any reason this is still a draft? Let's discuss the details over there.

Overall it sounds like the way forward!

[thorfour]

#202