From 89b258c5a78cfb560563c6e995b8aceda9ed1a04 Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Fri, 15 Mar 2019 21:25:07 -0700
Subject: [PATCH] ircd::fs: Improve fs::prefetch().

---
 include/ircd/fs/read.h |  2 +-
 ircd/fs.cc             | 33 +++++++++++++--------------------
 ircd/fs_aio.cc         | 12 ------------
 ircd/fs_aio.h          |  1 -
 4 files changed, 14 insertions(+), 34 deletions(-)

diff --git a/include/ircd/fs/read.h b/include/ircd/fs/read.h
index 5e8ee1144..fd2b760e4 100644
--- a/include/ircd/fs/read.h
+++ b/include/ircd/fs/read.h
@@ -28,7 +28,7 @@ namespace ircd::fs
 	std::string read(const string_view &path, const read_opts & = read_opts_default);
 
 	// Prefetch bytes for subsequent read(); offset is given in opts.
-	void prefetch(const fd &, const size_t &, const read_opts & = read_opts_default);
+	size_t prefetch(const fd &, const size_t &, const read_opts & = read_opts_default);
 }
 
 /// Options for a read operation
diff --git a/ircd/fs.cc b/ircd/fs.cc
index c4e795e8f..9ddb515b3 100644
--- a/ircd/fs.cc
+++ b/ircd/fs.cc
@@ -445,36 +445,29 @@ ircd::fs::read_opts
 const ircd::fs::read_opts_default
 {};
 
-#ifdef __linux__
-void
+size_t
 ircd::fs::prefetch(const fd &fd,
                    const size_t &count,
                    const read_opts &opts)
 {
-	const auto flags
+	static const size_t max_count
 	{
-		syscall(::fcntl, fd, F_GETFL)
+		128_KiB
 	};
 
-	if(~flags & O_DIRECT)
+	size_t i(0), off, cnt; do
 	{
-		syscall(::readahead, fd, opts.offset, count);
-		return;
+		off = opts.offset + max_count * i++;
+		cnt = std::min(opts.offset + count - off, max_count);
+		switch(const auto r(::posix_fadvise(fd, off, cnt, POSIX_FADV_WILLNEED)); r)
+		{
+			case 0:   break;
+			default:  throw_system_error(r);
+		}
 	}
-
-	#ifdef IRCD_USE_AIO
-	if(likely(aio::system) && opts.aio)
-		aio::prefetch(fd, count, opts);
-	#endif
+	while(off + cnt < opts.offset + count);
+	return count;
 }
-#else
-void
-ircd::fs::prefetch(const fd &fd,
-                   const size_t &count,
-                   const read_opts &opts)
-{
-}
-#endif
 
 std::string
 ircd::fs::read(const string_view &path,
diff --git a/ircd/fs_aio.cc b/ircd/fs_aio.cc
index 450ee5cb7..d89da9dc8 100644
--- a/ircd/fs_aio.cc
+++ b/ircd/fs_aio.cc
@@ -258,18 +258,6 @@ ircd::fs::aio::write(const fd &fd,
 	return bytes;
 }
 
-//
-// request::prefetch
-//
-
-void
-ircd::fs::aio::prefetch(const fd &fd,
-                        const size_t &size,
-                        const read_opts &opts)
-{
-
-}
-
 //
 // request
 //
diff --git a/ircd/fs_aio.h b/ircd/fs_aio.h
index 855a6722a..0c3b13fc8 100644
--- a/ircd/fs_aio.h
+++ b/ircd/fs_aio.h
@@ -17,7 +17,6 @@ namespace ircd::fs::aio
 	struct system;
 	struct request;
 
-	void prefetch(const fd &, const size_t &, const read_opts &);
 	size_t write(const fd &, const const_iovec_view &, const write_opts &);
 	size_t read(const fd &, const const_iovec_view &, const read_opts &);
 	void fdsync(const fd &, const sync_opts &);