Speedscope Profiles Dropped By Compaction: Causes & Fixes

by SLV Team 58 views
Speedscope Profiles Dropped by Compaction: Causes & Fixes

Have you ever encountered an issue where your Speedscope profiles seem to disappear over time in Grafana or Pyroscope? It's a frustrating problem, especially when you're trying to analyze performance and track down bottlenecks. This article dives deep into this bug, exploring the root cause and providing a potential solution. Let's get started, guys!

Understanding the Bug: Speedscope Profiles and Compaction

The Issue: Speedscope Profiles Vanishing Act

The core issue revolves around the fact that Speedscope profiles uploaded via the /ingest endpoint may not fully persist. Initially, the upload appears correct, but you might notice series (indicated by missing symbols in the UI) gradually disappearing as time goes on. This phenomenon occurs due to the compaction process, which is designed to optimize storage but, in this case, inadvertently drops crucial data. This issue has been observed and reported in the Grafana/Pyroscope ecosystem, highlighting the importance of understanding how profiles are handled during compaction.

Replicating the Bug: A Step-by-Step Guide

To reproduce the bug, follow these steps:

  1. Start Pyroscope on localhost:4040.

  2. Use the following curl command to upload a Speedscope profile:

    curl -X POST "localhost:4040/ingest?name=MyServiceName&from=$(gdate -d '6 minutes ago' +%s)&until=$(gdate -d '15 minutes ago' +%s)&format=speedscope" \
> -H "Content-Type: application/json" \
> -d @speedscope.json

  3. Observe the Thread (*) symbols at localhost:4040. Over time, you'll notice these symbols being dropped as compaction runs. This is the manifestation of the bug.

The above steps demonstrate a practical way to replicate the issue, enabling developers and users to witness the behavior firsthand and verify any potential fixes.

The Context: Where It All Started

This issue was originally reported in the Pyroscope GitHub repository, specifically in issue #3989. The detailed description and discussion in the issue provide valuable context and insights into the problem. Understanding the context helps in appreciating the efforts to resolve the issue and the importance of addressing it.

Diving into the Root Cause: The Hack That Reveals the Truth

The Diagnostic Diff: A Glimpse into the Problem

A crucial piece of the puzzle is a diff applied to the speedscope-ingestion-fix branch. This diff, while a hack, effectively resolves the issue and, more importantly, reveals the root cause. By examining the changes made in the diff, we can gain a deeper understanding of what's going wrong during the compaction process.

diff --git a/pkg/og/convert/speedscope/parser.go b/pkg/og/convert/speedscope/parser.go
index 5e0214912..a183c858d 100644
--- a/pkg/og/convert/speedscope/parser.go
+++ b/pkg/og/convert/speedscope/parser.go
@@ -4,6 +4,7 @@ import (
  	"context"
  	"encoding/json"
  	"fmt"
+	"time"
 
  	"github.com/grafana/pyroscope/pkg/og/ingestion"
  	"github.com/grafana/pyroscope/pkg/og/storage"
@@ -52,8 +53,8 @@ func parseAll(rawData []byte, md ingestion.Metadata) ([]*storage.PutInput, error
  	LabelSet:   md.LabelSet,
  }
 
- for _, prof := range file.Profiles {
-  putInput, err := parseOne(&prof, input, file.Shared.Frames, len(file.Profiles) > 1)
+ for i, prof := range file.Profiles {
+  putInput, err := parseOne(i, &prof, input, file.Shared.Frames, len(file.Profiles) > 1)
  if err != nil {
  return nil, err
  }
@@ -62,7 +63,7 @@ func parseAll(rawData []byte, md ingestion.Metadata) ([]*storage.PutInput, error
  return results, nil
 }
 
-func parseOne(prof *profile, putInput storage.PutInput, frames []frame, multi bool) (*storage.PutInput, error) {
+func parseOne(i int, prof *profile, putInput storage.PutInput, frames []frame, multi bool) (*storage.PutInput, error) {
  // Fixup some metadata
  putInput.Units = prof.Unit.chooseMetadataUnit()
  putInput.AggregationType = metadata.SumAggregationType
@@ -76,6 +77,8 @@ func parseOne(prof *profile, putInput storage.PutInput, frames []frame, multi bool) (*storage.PutInput, error) {
  putInput.SampleRate = uint32(prof.Unit.defaultSampleRate())
  }
 
+ putInput.StartTime = putInput.StartTime.Add(time.Second * time.Duration(i))
+
  var err error
  tr := tree.New()
  switch prof.Type {

The Key Change: Shifting Start Times

The most significant change in the diff is the addition of the line putInput.StartTime = putInput.StartTime.Add(time.Second * time.Duration(i)). This line modifies the start time of each profile by adding a small increment (in seconds) based on its index within the list of profiles. This seemingly minor adjustment has a profound impact on the persistence of Speedscope profiles.

Why Does This Fix It? Time-Based Differentiation

The reason this "hack" works is that it introduces a slight time-based differentiation between the profiles. Without this differentiation, the compaction process might incorrectly identify and discard profiles that appear to have overlapping or identical time ranges. By staggering the start times, even by a small amount, the profiles become distinguishable to the compaction logic, ensuring their preservation.

Implications and Solutions: What's Next?

The Underlying Problem: Compaction Logic

The "hack" provides a workaround, but it also shines a light on the underlying problem: the compaction logic in Pyroscope (or Grafana) needs to be more robust in handling Speedscope profiles with potentially overlapping time ranges. A more permanent solution would involve refining the compaction algorithm to accurately identify and preserve distinct profiles, even if they share similar time intervals.

Potential Solutions: Avenues for Improvement

  1. Refine Compaction Logic: Investigate and modify the compaction algorithm to better handle profiles with overlapping time ranges. This might involve more sophisticated time-based analysis or the introduction of additional criteria for differentiating profiles.
  2. Metadata Enrichment: Add more metadata to Speedscope profiles to provide additional context for the compaction process. This could include unique identifiers or other attributes that help distinguish profiles from each other.
  3. Profile Merging Strategies: Explore strategies for merging Speedscope profiles intelligently during compaction. This could involve combining profiles with similar characteristics while preserving the essential information.

Community Involvement: Let's Collaborate

The issue of Speedscope profiles being dropped by compaction highlights the importance of community involvement in open-source projects like Grafana and Pyroscope. By sharing experiences, reporting bugs, and contributing solutions, users and developers can collectively improve the stability and reliability of these tools. Let's work together to make profiling and performance analysis even better!

Conclusion: Keeping Your Profiles Safe and Sound

The case of disappearing Speedscope profiles is a valuable learning experience. It demonstrates how seemingly minor issues can have significant consequences and how a deep dive into the code can reveal the root cause. While the "hack" provides a temporary fix, the real solution lies in improving the compaction logic. By understanding the problem and exploring potential solutions, we can ensure that our Speedscope profiles remain safe and sound, allowing us to effectively analyze performance and optimize our applications. Keep profiling, guys, and let's build faster, more efficient software!