Author: glebius
Date: Thu Nov 17 21:36:18 2016
New Revision: 308784
URL: https://svnweb.freebsd.org/changeset/base/308784
Log:
Add flag SF_USER_READAHEAD to sendfile(2). When specified, the syscall won't
do any speculations about readahead, and use exactly the amount of readahead
specified by user. E.g. setting SF_FLAGS(0, SF_USER_READAHEAD) will guarantee
that no readahead at all will be performed.
Modified:
head/lib/libc/sys/sendfile.2
head/sys/kern/kern_sendfile.c
head/sys/sys/socket.h
Modified: head/lib/libc/sys/sendfile.2
==============================================================================
--- head/lib/libc/sys/sendfile.2 Thu Nov 17 21:02:55 2016
(r308783)
+++ head/lib/libc/sys/sendfile.2 Thu Nov 17 21:36:18 2016
(r308784)
@@ -25,7 +25,7 @@
.\"
.\" $FreeBSD$
.\"
-.Dd January 7, 2016
+.Dd November 17, 2016
.Dt SENDFILE 2
.Os
.Sh NAME
@@ -155,8 +155,33 @@ sleeps until the network stack no longer
of the file, making subsequent modifications to it safe.
Please note that this is not a guarantee that the data has actually
been sent.
+.It Dv SF_USER_READAHEAD
+.Nm
+has some internal heuristics to do readahead when sending data.
+This flag forces
+.Nm
+to override any heuristically calculated readahead and use exactly the
+application specified readahead.
+See
+.Sx SETTING READAHEAD
+for more details on readahead.
.El
.Pp
+When using a socket marked for non-blocking I/O,
+.Fn sendfile
+may send fewer bytes than requested.
+In this case, the number of bytes successfully
+written is returned in
+.Fa *sbytes
+(if specified),
+and the error
+.Er EAGAIN
+is returned.
+.Sh SETTING READAHEAD
+.Nm
+uses internal heuristics based on request size and file system layout
+to do readahead.
+Additionally application may request extra readahead.
The most significant 16 bits of
.Fa flags
specify amount of pages that
@@ -173,16 +198,13 @@ flag:
SF_FLAGS(16, SF_NOCACHE)
.Ed
.Pp
-When using a socket marked for non-blocking I/O,
-.Fn sendfile
-may send fewer bytes than requested.
-In this case, the number of bytes successfully
-written is returned in
-.Fa *sbytes
-(if specified),
-and the error
-.Er EAGAIN
-is returned.
+.Nm
+will use either application specified readahead or internally calculated,
+whichever is bigger.
+Setting flag
+.Dv SF_USER_READAHEAD
+would turn off any heuristics and set maximum possible readahead length to
+the number of pages specified via flags.
.Sh IMPLEMENTATION NOTES
The
.Fx
Modified: head/sys/kern/kern_sendfile.c
==============================================================================
--- head/sys/kern/kern_sendfile.c Thu Nov 17 21:02:55 2016
(r308783)
+++ head/sys/kern/kern_sendfile.c Thu Nov 17 21:36:18 2016
(r308784)
@@ -706,13 +706,20 @@ retry_space:
/*
* Calculate maximum allowed number of pages for readahead
- * at this iteration. First, we allow readahead up to "rem".
+ * at this iteration. If SF_USER_READAHEAD was set, we don't
+ * do any heuristics and use exactly the value supplied by
+ * application. Otherwise, we allow readahead up to "rem".
* If application wants more, let it be, but there is no
* reason to go above MAXPHYS. Also check against "obj_size",
* since vm_pager_has_page() can hint beyond EOF.
*/
- rhpages = howmany(rem + (off & PAGE_MASK), PAGE_SIZE) - npages;
- rhpages += SF_READAHEAD(flags);
+ if (flags & SF_USER_READAHEAD) {
+ rhpages = SF_READAHEAD(flags);
+ } else {
+ rhpages = howmany(rem + (off & PAGE_MASK), PAGE_SIZE) -
+ npages;
+ rhpages += SF_READAHEAD(flags);
+ }
rhpages = min(howmany(MAXPHYS, PAGE_SIZE), rhpages);
rhpages = min(howmany(obj_size - trunc_page(off), PAGE_SIZE) -
npages, rhpages);
Modified: head/sys/sys/socket.h
==============================================================================
--- head/sys/sys/socket.h Thu Nov 17 21:02:55 2016 (r308783)
+++ head/sys/sys/socket.h Thu Nov 17 21:36:18 2016 (r308784)
@@ -590,6 +590,7 @@ struct sf_hdtr {
#define SF_NODISKIO 0x00000001
#define SF_MNOWAIT 0x00000002 /* obsolete */
#define SF_SYNC 0x00000004
+#define SF_USER_READAHEAD 0x00000008
#define SF_NOCACHE 0x00000010
#define SF_FLAGS(rh, flags) (((rh) << 16) | (flags))
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