From 9814868e19a67ab7e0ab49a8dcf830e589a7f5ef Mon Sep 17 00:00:00 2001
From: Carl Schwan <carl@carlschwan.eu>
Date: Mon, 5 Oct 2026 14:44:02 +0200
Subject: [PATCH] perf(preview): cache the hardware concurrency

Every preview request reads /proc/cpuinfo twice to compute the default
preview concurrency. The kernel generates that file on each read, which
costs as much as looking up a cached preview. Cache the value in the
local cache instead.

Assisted-by: ClaudeCode:claude-opus-5-5
Signed-off-by: Carl Schwan <carl@carlschwan.eu>
---
 lib/private/Preview/Generator.php   | 18 ++++++++++++++++--
 lib/private/PreviewManager.php      |  2 ++
 tests/lib/Preview/GeneratorTest.php | 22 ++++++++++++++++++++++
 3 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/lib/private/Preview/Generator.php b/lib/private/Preview/Generator.php
index 94fea7314b565..8c0bd6f24ce7f 100644
--- a/lib/private/Preview/Generator.php
+++ b/lib/private/Preview/Generator.php
@@ -21,6 +21,7 @@
 use OCP\Files\SimpleFS\InMemoryFile;
 use OCP\Files\SimpleFS\ISimpleFile;
 use OCP\IAppConfig;
+use OCP\ICacheFactory;
 use OCP\IConfig;
 use OCP\IImage;
 use OCP\Image;
@@ -46,6 +47,7 @@ public function __construct(
 		private readonly PreviewMapper $previewMapper,
 		private readonly StorageFactory $storageFactory,
 		private readonly PreviewMigrationService $migrationService,
+		private readonly ICacheFactory $cacheFactory,
 	) {
 	}
 
@@ -261,7 +263,19 @@ public static function unguardWithSemaphore(false|\SysvSemaphore $semId): bool {
 	 *
 	 * @return int number of concurrent threads, or 0 if it cannot be determined
 	 */
-	private static function getHardwareConcurrency(): int {
+	private function getHardwareConcurrency(): int {
+		// /proc/cpuinfo is slow to read
+		$cache = $this->cacheFactory->createLocal('preview');
+		$cached = $cache->get('hardware_concurrency');
+		if (is_int($cached)) {
+			return $cached;
+		}
+		$concurrency = self::readHardwareConcurrency();
+		$cache->set('hardware_concurrency', $concurrency, 3600);
+		return $concurrency;
+	}
+
+	private static function readHardwareConcurrency(): int {
 		if (function_exists('ini_get')) {
 			$openBasedir = ini_get('open_basedir');
 			if (empty($openBasedir) || strpos($openBasedir, '/proc/cpuinfo') !== false) {
@@ -291,7 +305,7 @@ public function getNumConcurrentPreviews(string $type): int {
 			return $this->cachedNumConcurrentPreviews[$type];
 		}
 
-		$hardwareConcurrency = self::getHardwareConcurrency();
+		$hardwareConcurrency = $this->getHardwareConcurrency();
 		switch ($type) {
 			case 'preview_concurrency_all':
 				$fallback = $hardwareConcurrency > 0 ? $hardwareConcurrency * 2 : 8;
diff --git a/lib/private/PreviewManager.php b/lib/private/PreviewManager.php
index c3f780de39ecb..69b74f032fc83 100644
--- a/lib/private/PreviewManager.php
+++ b/lib/private/PreviewManager.php
@@ -58,6 +58,7 @@
 use OCP\Files\SimpleFS\ISimpleFile;
 use OCP\IAppConfig;
 use OCP\IBinaryFinder;
+use OCP\ICacheFactory;
 use OCP\IConfig;
 use OCP\IPreview;
 use OCP\Preview\IProviderV2;
@@ -161,6 +162,7 @@ private function getGenerator(): Generator {
 				$this->container->get(PreviewMapper::class),
 				$this->container->get(StorageFactory::class),
 				$this->container->get(PreviewMigrationService::class),
+				$this->container->get(ICacheFactory::class),
 			);
 		}
 		return $this->generator;
diff --git a/tests/lib/Preview/GeneratorTest.php b/tests/lib/Preview/GeneratorTest.php
index 1b62768ef390d..526ac19cff10b 100644
--- a/tests/lib/Preview/GeneratorTest.php
+++ b/tests/lib/Preview/GeneratorTest.php
@@ -483,6 +483,28 @@ public function testCorrectSize(int $maxX, int $maxY, int $reqX, int $reqY, bool
 		}
 	}
 
+	public function testHardwareConcurrencyFromCache(): void {
+		$this->getAutoMock(IConfig::class)->method('getSystemValueInt')
+			->willReturnCallback(fn ($key, $default) => $default);
+		$cache = $this->getCacheAutoMock('preview');
+		$cache->method('get')->with('hardware_concurrency')->willReturn(3);
+		$cache->expects($this->never())->method('set');
+
+		$this->assertSame(6, $this->generator->getNumConcurrentPreviews('preview_concurrency_all'));
+		$this->assertSame(3, $this->generator->getNumConcurrentPreviews('preview_concurrency_new'));
+	}
+
+	public function testHardwareConcurrencyIsCached(): void {
+		$this->getAutoMock(IConfig::class)->method('getSystemValueInt')
+			->willReturnCallback(fn ($key, $default) => $default);
+		$cache = $this->getCacheAutoMock('preview');
+		$cache->method('get')->with('hardware_concurrency')->willReturn(null);
+		$cache->expects($this->once())->method('set')
+			->with('hardware_concurrency', $this->isInt(), 3600);
+
+		$this->generator->getNumConcurrentPreviews('preview_concurrency_new');
+	}
+
 	public function testUnreadbleFile(): void {
 		$file = $this->createMock(File::class);
 		$file->method('isReadable')