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Date: Sun, 21 Feb 2010 22:26:00 +0800
From: Wu Fengguang <fengguang.wu@...el.com>
To: Nikanth Karthikesan <knikanth@...e.de>
Cc: Dave Chinner <david@...morbit.com>, Ankit Jain <radical@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
Jens Axboe <jens.axboe@...cle.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] Make VM_MAX_READAHEAD a kernel parameter
Nikanth,
> > > + readahead= Default readahead value for block devices.
> > > +
> >
> > I think the description should define the units (kb) and valid value
> > ranges e.g. page size to something not excessive - say 65536kb. The
> > above description is, IMO, useless without refering to the source to
> > find out this information....
> >
>
> The parameter can be specified with/without any suffix(k/m/g) that memparse()
> helper function can accept. So it can take 1M, 1024k, 1050620. I checked other
> parameters that use memparse() to get similar values and they didn't document
> it. May be this should be described here.
Hope this helps clarify things to user:
+ readahead=nn[KM]
+ Default max readahead size for block devices.
+ Range: 0; 4k - 128m
> > And readahead_kb needs to be validated against the range of
> > valid values here.
> >
>
> I didn't want to impose artificial restrictions. I think Wu's patch set would
> be adding some restrictions, like minimum readahead. He could fix it when he
> modifies the patch to include in his patch set.
OK, I imposed a larger bound -- 128MB.
And values 1-4095 (more exactly: PAGE_CACHE_SIZE) are prohibited mainly to
catch "readahead=128" where the user really means to do 128 _KB_ readahead.
Christian, with this patch and more patches to scale down readahead
size on small memory/device size, I guess it's no longer necessary to
introduce a CONFIG_READAHEAD_SIZE?
Thanks,
Fengguang
---
make default readahead size a kernel parameter
From: Nikanth Karthikesan <knikanth@...e.de>
Add new kernel parameter "readahead", which would be used instead of the
value of VM_MAX_READAHEAD. If the parameter is not specified, the default
of 128kb would be used.
CC: Ankit Jain <radical@...il.com>
CC: Dave Chinner <david@...morbit.com>
CC: Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Signed-off-by: Nikanth Karthikesan <knikanth@...e.de>
Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
Documentation/kernel-parameters.txt | 4 ++++
block/blk-core.c | 3 +--
fs/fuse/inode.c | 2 +-
include/linux/mm.h | 2 ++
mm/readahead.c | 26 ++++++++++++++++++++++++++
5 files changed, 34 insertions(+), 3 deletions(-)
--- linux.orig/Documentation/kernel-parameters.txt 2010-02-21 22:09:41.000000000 +0800
+++ linux/Documentation/kernel-parameters.txt 2010-02-21 22:11:08.000000000 +0800
@@ -2174,6 +2174,10 @@ and is between 256 and 4096 characters.
Run specified binary instead of /init from the ramdisk,
used for early userspace startup. See initrd.
+ readahead=nn[KM]
+ Default max readahead size for block devices.
+ Range: 0; 4k - 128m
+
reboot= [BUGS=X86-32,BUGS=ARM,BUGS=IA-64] Rebooting mode
Format: <reboot_mode>[,<reboot_mode2>[,...]]
See arch/*/kernel/reboot.c or arch/*/kernel/process.c
--- linux.orig/block/blk-core.c 2010-02-21 22:09:41.000000000 +0800
+++ linux/block/blk-core.c 2010-02-21 22:09:42.000000000 +0800
@@ -498,8 +498,7 @@ struct request_queue *blk_alloc_queue_no
q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
q->backing_dev_info.unplug_io_data = q;
- q->backing_dev_info.ra_pages =
- (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+ q->backing_dev_info.ra_pages = max_readahead_pages;
q->backing_dev_info.state = 0;
q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
q->backing_dev_info.name = "block";
--- linux.orig/fs/fuse/inode.c 2010-02-21 22:09:41.000000000 +0800
+++ linux/fs/fuse/inode.c 2010-02-21 22:09:42.000000000 +0800
@@ -870,7 +870,7 @@ static int fuse_bdi_init(struct fuse_con
int err;
fc->bdi.name = "fuse";
- fc->bdi.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+ fc->bdi.ra_pages = max_readahead_pages;
fc->bdi.unplug_io_fn = default_unplug_io_fn;
/* fuse does it's own writeback accounting */
fc->bdi.capabilities = BDI_CAP_NO_ACCT_WB;
--- linux.orig/include/linux/mm.h 2010-02-21 22:09:41.000000000 +0800
+++ linux/include/linux/mm.h 2010-02-21 22:09:42.000000000 +0800
@@ -1187,6 +1187,8 @@ void task_dirty_inc(struct task_struct *
#define VM_MAX_READAHEAD 128 /* kbytes */
#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
+extern unsigned long max_readahead_pages;
+
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read);
--- linux.orig/mm/readahead.c 2010-02-21 22:09:41.000000000 +0800
+++ linux/mm/readahead.c 2010-02-21 22:13:44.000000000 +0800
@@ -19,6 +19,32 @@
#include <linux/pagevec.h>
#include <linux/pagemap.h>
+unsigned long max_readahead_pages = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE;
+
+static int __init readahead(char *str)
+{
+ unsigned long bytes;
+
+ if (!str)
+ return -EINVAL;
+ bytes = memparse(str, &str);
+ if (*str != '\0')
+ return -EINVAL;
+
+ if (bytes) {
+ if (bytes < PAGE_CACHE_SIZE) /* missed 'k'/'m' suffixes? */
+ return -EINVAL;
+ if (bytes > 128 << 20) /* limit to 128MB */
+ bytes = 128 << 20;
+ }
+
+ max_readahead_pages = bytes / PAGE_CACHE_SIZE;
+ default_backing_dev_info.ra_pages = max_readahead_pages;
+ return 0;
+}
+
+early_param("readahead", readahead);
+
/*
* Initialise a struct file's readahead state. Assumes that the caller has
* memset *ra to zero.
--
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