ClickHouse: Cold runs preload data into memory before the timer

未關閉
  • 哪裡不對？
#941 1 則留言 5 個 reaction 已指派 0 人 在 GitHub 檢視

維護者通常 1 天內回覆

還沒有人認領這個 Issue。

評估

難度
4/5
預估耗時
3-5 天
新手友好度
45/100
Issue 類型
缺陷
描述清晰度
基本清楚
活躍度
冷清
技術堆疊
shell, sql
領域
databases, performance

研究方向

先閱讀 README.md 中對 true-cold 的定義，以及 ClickHouse/install 的 startup 註解。追蹤 startup 載入與快取清除在 benchmark 流程中是如何表示的。只有在明確決定 cold-run 策略，且 benchmark 行為與文件一致後，才算完成。

由索引模型根據 Issue 內容生成。

描述

Commit 7c1f7a3291521738a7731af38c994debb1ea3b1e enables preloading of the primary key on ClickHouse startup.

Quoting clickhouse/install:

# Force synchronous startup loading so the cold timer doesn't catch
# work that should have been amortized into ./start.
#
# Two independent layers of laziness contribute to the cold-query floor:
#
# 1. async_load_databases (server-level): with the default 1, the server
#    binds its listen port and answers SELECT 1 before user databases
#    have finished loading. ./check passes, then the first query stalls
#    waiting for the part loader.
#
# 2. primary_key_lazy_load and columns_and_secondary_indices_sizes_lazy_calculation
#    (MergeTree-level): even after parts are loaded, the in-memory
#    primary key and the per-column .size streams are populated lazily
#    on first query. With ~25 parts × ~80 columns × multiple metadata
#    files per column that's >1.8k file opens on the first query path,
#    contributing several hundred ms even on local NVMe.
#
# Both are eager-load toggles, not caching shortcuts: the same I/O
# happens either way, just before query timing instead of during it.
# Together they brought Q40 cold from ~3 s to ~1.5 s on c6a.4xlarge.

README.md defines true cold runs differently:

2.a) True cold runs. Before each first run of each query, all [...] database caches (e.g. buffer pools) are cleared.

In the current configuration, ClickHouse pre-warms the primary key data at startup, which defeats the clearing of all database caches. The measured cold runs times aren't truly cold anymore. While size-calculations might be considered metadata, primary key data is definitively data that's also queried by the benchmark queries.

I'm not sure there's a good definition which data is fine to be cached for a true cold run and which not.
Two of my suggestions below:

  1. Include startup times in "true cold" runs. This effectively prevents cheating by preloading data at startup.
  2. Drop the cold run altogether. "Cold" is clearly different for different systems and does not compare performance apples-to-apples
主要語言
Shell
星號
1.2k
分支
330
平均合併
4 小時 12 分鐘
30 天內合併 PR
605

環境準備

  • 沒有 Dockerfile 或 Docker Compose 檔案
  • 有 Pull Request 範本
  • 沒有貢獻指南

從這裡開始

  1. 先讀完整個 Issue，再讀專案的貢獻指南。
  2. 在 Issue 下留言說明你要接手 —— 這能避免兩個人做同樣的事。
  3. Fork 儲存庫，在一個分支上完成修改。
  4. 送出 Pull Request，並在描述裡引用這個 Issue 編號。

ClickHouse/ClickBench 的其他 Issue

查看 ClickHouse/ClickBench 的全部 Issue

相似的 Issue

更多 Shell/Bash Issue

把新 issue 寄到你的電子郵件信箱

精選適合新手參與的 GitHub issue 摘要。