From 9dca9f2bcd06f921312762e0b07254f5f8f51fc2 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Tue, 23 May 2023 14:34:07 -0400
Subject: eval: Pre-allocate the highest fd
This avoids the need to grow the fd table during the search,
significantly reducing kernel contention when opening directories in
parallel.
---
src/eval.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
(limited to 'src')
diff --git a/src/eval.c b/src/eval.c
index b9bce6c..f16821e 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1471,6 +1471,23 @@ static int raise_fdlimit(const struct bfs_ctx *ctx) {
return ret;
}
+/** Preallocate the fd table in the kernel. */
+static void reserve_fds(int limit) {
+ // Kernels typically implement the fd table as a dynamic array.
+ // Growing the array can be expensive, especially if files are being
+ // opened in parallel. We can work around this by allocating the
+ // highest possible fd, forcing the kernel to grow the table upfront.
+
+#ifdef F_DUPFD_CLOEXEC
+ int fd = fcntl(STDIN_FILENO, F_DUPFD_CLOEXEC, limit - 1);
+#else
+ int fd = fcntl(STDIN_FILENO, F_DUPFD, limit - 1);
+#endif
+ if (fd >= 0) {
+ xclose(fd);
+ }
+}
+
/** Infer the number of file descriptors available to bftw(). */
static int infer_fdlimit(const struct bfs_ctx *ctx, int limit) {
// 3 for std{in,out,err}
@@ -1604,6 +1621,7 @@ int bfs_eval(const struct bfs_ctx *ctx) {
}
int fdlimit = raise_fdlimit(ctx);
+ reserve_fds(fdlimit);
fdlimit = infer_fdlimit(ctx, fdlimit);
int nthreads;
--
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