[C++] Vtable Deduplication Fails For 64-bit Buffers >2GB, Causing Slow Serialization And Larger Buffers

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Introduction

In the realm of C++ and FlatBuffers, vtable deduplication is a crucial optimization technique that helps reduce memory usage and improve performance. However, when dealing with 64-bit buffers larger than 2GB, vtable deduplication fails due to incorrect offset calculations. This results in slower serialization times and larger-than-expected buffer sizes. In this article, we will delve into the issue, explore the steps to reproduce it, and discuss the expected and actual behavior.

Description

When serializing FlatBuffers with 64-bit offsets for buffers larger than 2GB, vtable deduplication fails due to incorrect offset calculations. This leads to significantly slower serialization times and larger-than-expected buffer sizes due to redundant vtables. Disabling vtable deduplication using fbb.DedupVtables(false) mitigates the issue, which pinpoints the problem to the deduplication logic itself.

Steps to Reproduce

To reproduce this issue, follow these steps:

  1. Enable 64-bit offsets in FlatBuffers: Make sure that your FlatBuffers configuration enables 64-bit offsets. This is typically done by setting the offset_base option to 64.
  2. Serialize a buffer greater than 2GB containing multiple objects with identical layouts: Create a buffer that exceeds 2GB in size and contains multiple objects with identical layouts. This will trigger the vtable deduplication logic.

Observe

After reproducing the issue, observe the following:

  • Serialization of large buffers (over 2GB) takes much longer than for smaller buffers: The serialization process will take significantly longer for large buffers compared to smaller ones.
  • The resulting buffer files are noticeably larger than expected when serializing many objects with identical layouts: The resulting buffer files will be larger than expected due to the lack of vtable deduplication.
  • Disabling vtable deduplication with fbb.DedupVtables(false) significantly reduces serialization time, but results in even larger buffer files: Disabling vtable deduplication will improve the serialization time, but the resulting buffer files will be even larger.

Expected Behavior

The expected behavior is:

  • Vtables should deduplicate efficiently, even for 64-bit buffers: Vtable deduplication should work efficiently even for 64-bit buffers.
  • Serialization time and buffer size should scale reasonably with data size: The serialization time and buffer size should scale reasonably with the data size.

Actual Behavior

The actual behavior is:

  • Serialization is much slower for large buffers using 64-bit offsets: Serialization is much slower for large buffers using 64-bit offsets.
  • Buffer size increases due to lack of vtable deduplication: The buffer size increases due to the lack of vtable deduplication.
  • Disabling vtable deduplication improves speed but increases buffer size further: Disabling vtable deduplication improves the speed, but the buffer size increases further.

Environment

The environment in which this issue occurs is:

  • FlatBuffers version: 25.2.10: The issue occurs in FlatBuffers version 25.2.10.
  • Platform: any platform/compiler with support for 64-bit offsets: The issue occurs on any platform or compiler that supports 64-bit offsets.

Additional Context

The core issue stems from 64-bit offset calculations omitting length_of_64_bit_region_. This exclusion causes vtable candidate addresses to be miscalculated, which prevents deduplication. A fix that includes length_of_64_bit_region_ in the offset calculation restores correct vtable deduplication and improves serialization performance for large buffers.

Conclusion

In conclusion, vtable deduplication fails for 64-bit buffers larger than 2GB due to incorrect offset calculations. This results in slower serialization times and larger-than-expected buffer sizes. Disabling vtable deduplication mitigates the issue, but a fix that includes length_of_64_bit_region_ in the offset calculation restores correct vtable deduplication and improves serialization performance for large buffers.

Recommendations

To avoid this issue, follow these recommendations:

  • Use 32-bit offsets for buffers smaller than 2GB: Use 32-bit offsets for buffers smaller than 2GB to avoid the issue.
  • Disable vtable deduplication for large buffers: Disable vtable deduplication for large buffers to improve serialization performance.
  • Apply the fix that includes length_of_64_bit_region_ in the offset calculation: Apply the fix that includes length_of_64_bit_region_ in the offset calculation to restore correct vtable deduplication and improve serialization performance for large buffers.

Introduction

In our previous article, we discussed the issue of vtable deduplication failing for 64-bit buffers larger than 2GB, causing slow serialization and larger buffers. In this article, we will provide a Q&A section to address common questions and concerns related to this issue.

Q: What is vtable deduplication, and why is it important?

A: Vtable deduplication is a technique used in C++ to reduce memory usage and improve performance by eliminating redundant vtables. Vtables are tables of function pointers that are used to implement virtual functions in C++. By deduplicating vtables, we can reduce the memory footprint of our applications and improve their performance.

Q: What causes vtable deduplication to fail for 64-bit buffers larger than 2GB?

A: The core issue stems from 64-bit offset calculations omitting length_of_64_bit_region_. This exclusion causes vtable candidate addresses to be miscalculated, which prevents deduplication.

Q: What are the symptoms of vtable deduplication failing for 64-bit buffers larger than 2GB?

A: The symptoms of vtable deduplication failing for 64-bit buffers larger than 2GB include:

  • Serialization of large buffers (over 2GB) takes much longer than for smaller buffers.
  • The resulting buffer files are noticeably larger than expected when serializing many objects with identical layouts.
  • Disabling vtable deduplication with fbb.DedupVtables(false) significantly reduces serialization time, but results in even larger buffer files.

Q: How can I reproduce the issue?

A: To reproduce the issue, follow these steps:

  1. Enable 64-bit offsets in FlatBuffers.
  2. Serialize a buffer greater than 2GB containing multiple objects with identical layouts.

Q: What is the expected behavior of vtable deduplication for 64-bit buffers larger than 2GB?

A: The expected behavior of vtable deduplication for 64-bit buffers larger than 2GB is:

  • Vtables should deduplicate efficiently, even for 64-bit buffers.
  • Serialization time and buffer size should scale reasonably with data size.

Q: What is the actual behavior of vtable deduplication for 64-bit buffers larger than 2GB?

A: The actual behavior of vtable deduplication for 64-bit buffers larger than 2GB is:

  • Serialization is much slower for large buffers using 64-bit offsets.
  • Buffer size increases due to lack of vtable deduplication.
  • Disabling vtable deduplication improves speed but increases buffer size further.

Q: How can I fix the issue of vtable deduplication failing for 64-bit buffers larger than 2GB?

A: To fix the issue of vtable deduplication failing for 64-bit buffers larger than 2GB, apply the fix that includes length_of_64_bit_ in the offset calculation.

Q: What are the recommendations for avoiding the issue of vtable deduplication failing for 64-bit buffers larger than 2GB?

A: The recommendations for avoiding the issue of vtable deduplication failing for 64-bit buffers larger than 2GB are:

  • Use 32-bit offsets for buffers smaller than 2GB.
  • Disable vtable deduplication for large buffers.
  • Apply the fix that includes length_of_64_bit_region_ in the offset calculation.

By following these recommendations, you can avoid the issue of vtable deduplication failing for 64-bit buffers larger than 2GB and improve the performance of your FlatBuffers serialization.

Conclusion

In conclusion, vtable deduplication fails for 64-bit buffers larger than 2GB due to incorrect offset calculations. This results in slower serialization times and larger-than-expected buffer sizes. By understanding the issue, its symptoms, and the recommended fixes, you can improve the performance of your FlatBuffers serialization and avoid the issue of vtable deduplication failing for 64-bit buffers larger than 2GB.