From 2a3cfc96656d85f42fa01537af93fd2ead1d9ca5 Mon Sep 17 00:00:00 2001
From: John Khvatov <ivaxer@yandex-team.com>
Date: Mon, 15 Jan 2024 19:19:19 +0300
Subject: [PATCH] Remove PAGE_CACHE_ACQUIRE_PINNED_SLOT_TIME histogram. (#6356)

Fixes #6343.

## Problem

PAGE_CACHE_ACQUIRE_PINNED_SLOT_TIME is used on hot path and it adds
noticeable latency to GetPage@LSN.

## Refs

https://discordapp.com/channels/1176467419317940276/1195022264115151001/1196370689268125716
---
 pageserver/src/metrics.rs    | 9 ---------
 pageserver/src/page_cache.rs | 2 --
 2 files changed, 11 deletions(-)

diff --git a/pageserver/src/metrics.rs b/pageserver/src/metrics.rs
index 6f4431c3cf..07f9049ca5 100644
--- a/pageserver/src/metrics.rs
+++ b/pageserver/src/metrics.rs
@@ -337,15 +337,6 @@ pub(crate) mod page_cache_eviction_metrics {
     }
 }
 
-pub(crate) static PAGE_CACHE_ACQUIRE_PINNED_SLOT_TIME: Lazy<Histogram> = Lazy::new(|| {
-    register_histogram!(
-        "pageserver_page_cache_acquire_pinned_slot_seconds",
-        "Time spent acquiring a pinned slot in the page cache",
-        CRITICAL_OP_BUCKETS.into(),
-    )
-    .expect("failed to define a metric")
-});
-
 static PAGE_CACHE_ERRORS: Lazy<IntCounterVec> = Lazy::new(|| {
     register_int_counter_vec!(
         "page_cache_errors_total",
diff --git a/pageserver/src/page_cache.rs b/pageserver/src/page_cache.rs
index c3c98af406..28d2584bf4 100644
--- a/pageserver/src/page_cache.rs
+++ b/pageserver/src/page_cache.rs
@@ -550,7 +550,6 @@ impl PageCache {
     // not require changes.
 
     async fn try_get_pinned_slot_permit(&self) -> anyhow::Result<PinnedSlotsPermit> {
-        let timer = crate::metrics::PAGE_CACHE_ACQUIRE_PINNED_SLOT_TIME.start_timer();
         match tokio::time::timeout(
             // Choose small timeout, neon_smgr does its own retries.
             // https://neondb.slack.com/archives/C04DGM6SMTM/p1694786876476869
@@ -563,7 +562,6 @@ impl PageCache {
                 res.expect("this semaphore is never closed"),
             )),
             Err(_timeout) => {
-                timer.stop_and_discard();
                 crate::metrics::page_cache_errors_inc(
                     crate::metrics::PageCacheErrorKind::AcquirePinnedSlotTimeout,
                 );