On Mon, Jul 23, 2007 at 12:40:09PM -0700, Andrew Morton wrote:
> On Mon, 23 Jul 2007 22:24:57 +0800
> Fengguang Wu <[email protected]> wrote:
>
> > On Mon, Jul 23, 2007 at 07:00:59PM +1000, Nick Piggin wrote:
> > > Rusty Russell wrote:
> > > >On Sun, 2007-07-22 at 16:10 +0800, Fengguang Wu wrote:
> > >
> > > >>So I opt for it being made tunable, safe, and turned off by default.
> > >
> > > I hate tunables :) Unless we have workload A that gets a reasonable
> > > benefit from something and workload B that gets a significant regression,
> > > and no clear way to reconcile them...
> >
> > Me too ;)
> >
> > But sometimes we really want to avoid flushing the cache.
> > Andrew's user space LD_PRELOAD+fadvise based tool fit nicely here.
>
> It's the only way to go in some situations. Sometimes the kernel just
> cannot predict the future sufficiently well, and the costs of making a
> mistake are terribly high. We need human help. And it should be
> administration-time help, not programming-time help.
Agreed. I feel that drop behind is not a universal applicable.
Cost based reclaim sounds better, but who knows before field use ;)
> > > >I'd like to see it turned on by default in -mm, and try to come up with
> > > >some server-like workload to measure the effect. Should be easy to
> > > >simulate something (eg. apache server, where clients grab some files in
> > > >preference, and apache server where clients grab different files).
> > >
> > > I don't like this kind of conditional information going from something
> > > like readahead into page reclaim. Unless it is for readahead _specific_
> > > data such as "I got these all wrong, so you can reclaim them" (which
> > > this isn't).
> > >
> > > Possibly it makes sense to realise that the given pages are cheaper
> > > to read back in as they are apparently being read-ahead very nicely.
> >
> > In fact I have talked to Jens about it in last year's kernel summit.
> > The patch below explains itself.
> > ---
> > Subject: cost based page reclaim
> >
> > Cost based page reclaim - a minimalist implementation.
> >
> > Suppose we cached 32 small files each with 1 page, and one 32-page chunk from a
> > large file. Should we first drop the 32-pages which are read in one I/O, or
> > drop the 32 distinct pages, each costs one I/O? (Given that the files are of
> > equal hotness.)
> >
> > Page replacement algorithms should be designed to minimize the number of I/Os,
> > instead of the number of page faults. Dividing the cost of I/O by the number of
> > pages it bring in, we get the cost of the page. The bigger page cost, the more
> > 'lives/bloods' the page should have.
> >
> > This patch adds life to costly pages by pretending that they are
> > referenced more times. Possible downsides:
> > - burdens the pressure of vmscan
> > - active pages are no longer that 'active'
> >
>
> This is all fun stuff, but how do we find out that changes like this are
> good ones, apart from shipping it and seeing who gets hurt 12 months later?
One thing I can imagine now is that the first pages may get more life
because of the conservative initial readahead size.
Generally file servers use sendfile(wholefile), so not a problem.
> > +#define log2(n) fls(n)
>
> <look at include/linux/log2.h>
Thank you, this comment lead to another patch :)
---
Subject: convert ill defined log2() to ilog2()
It's *wrong* to have
#define log2(n) ffz(~(n))
It should be *reversed*:
#define log2(n) flz(~(n))
or
#define log2(n) fls(n)
or just use
ilog2(n) defined in linux/log2.h.
This patch follows the last solution, recommended by Andrew Morton.
//Or are they simply the wrong naming, and is in fact correct in behavior?
Cc: [email protected]
Cc: Mingming Cao <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Chris Ahna <[email protected]>
Cc: David Mosberger-Tang <[email protected]>
Cc: Kyle McMartin <[email protected]>
Signed-off-by: Fengguang Wu <[email protected]>
---
drivers/char/agp/hp-agp.c | 9 +++------
drivers/char/agp/i460-agp.c | 5 ++---
drivers/char/agp/parisc-agp.c | 7 ++-----
fs/ext4/super.c | 6 ++----
4 files changed, 9 insertions(+), 18 deletions(-)
--- linux-2.6.22-rc6-mm1.orig/drivers/char/agp/hp-agp.c
+++ linux-2.6.22-rc6-mm1/drivers/char/agp/hp-agp.c
@@ -14,15 +14,12 @@
#include <linux/pci.h>
#include <linux/init.h>
#include <linux/agp_backend.h>
+#include <linux/log2.h>
#include <asm/acpi-ext.h>
#include "agp.h"
-#ifndef log2
-#define log2(x) ffz(~(x))
-#endif
-
#define HP_ZX1_IOC_OFFSET 0x1000 /* ACPI reports SBA, we want IOC */
/* HP ZX1 IOC registers */
@@ -256,7 +253,7 @@ hp_zx1_configure (void)
readl(hp->ioc_regs+HP_ZX1_IMASK);
writel(hp->iova_base|1, hp->ioc_regs+HP_ZX1_IBASE);
readl(hp->ioc_regs+HP_ZX1_IBASE);
- writel(hp->iova_base|log2(HP_ZX1_IOVA_SIZE), hp->ioc_regs+HP_ZX1_PCOM);
+ writel(hp->iova_base|ilog2(HP_ZX1_IOVA_SIZE), hp->ioc_regs+HP_ZX1_PCOM);
readl(hp->ioc_regs+HP_ZX1_PCOM);
}
@@ -284,7 +281,7 @@ hp_zx1_tlbflush (struct agp_memory *mem)
{
struct _hp_private *hp = &hp_private;
- writeq(hp->gart_base | log2(hp->gart_size), hp->ioc_regs+HP_ZX1_PCOM);
+ writeq(hp->gart_base | ilog2(hp->gart_size), hp->ioc_regs+HP_ZX1_PCOM);
readq(hp->ioc_regs+HP_ZX1_PCOM);
}
--- linux-2.6.22-rc6-mm1.orig/drivers/char/agp/i460-agp.c
+++ linux-2.6.22-rc6-mm1/drivers/char/agp/i460-agp.c
@@ -13,6 +13,7 @@
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/agp_backend.h>
+#include <linux/log2.h>
#include "agp.h"
@@ -59,8 +60,6 @@
*/
#define WR_FLUSH_GATT(index) RD_GATT(index)
-#define log2(x) ffz(~(x))
-
static struct {
void *gatt; /* ioremap'd GATT area */
@@ -148,7 +147,7 @@ static int i460_fetch_size (void)
* values[i].size.
*/
values[i].num_entries = (values[i].size << 8) >> (I460_IO_PAGE_SHIFT - 12);
- values[i].page_order = log2((sizeof(u32)*values[i].num_entries) >> PAGE_SHIFT);
+ values[i].page_order = ilog2((sizeof(u32)*values[i].num_entries) >> PAGE_SHIFT);
}
for (i = 0; i < agp_bridge->driver->num_aperture_sizes; i++) {
--- linux-2.6.22-rc6-mm1.orig/drivers/char/agp/parisc-agp.c
+++ linux-2.6.22-rc6-mm1/drivers/char/agp/parisc-agp.c
@@ -18,6 +18,7 @@
#include <linux/init.h>
#include <linux/klist.h>
#include <linux/agp_backend.h>
+#include <linux/log2.h>
#include <asm-parisc/parisc-device.h>
#include <asm-parisc/ropes.h>
@@ -27,10 +28,6 @@
#define DRVNAME "quicksilver"
#define DRVPFX DRVNAME ": "
-#ifndef log2
-#define log2(x) ffz(~(x))
-#endif
-
#define AGP8X_MODE_BIT 3
#define AGP8X_MODE (1 << AGP8X_MODE_BIT)
@@ -92,7 +89,7 @@ parisc_agp_tlbflush(struct agp_memory *m
{
struct _parisc_agp_info *info = &parisc_agp_info;
- writeq(info->gart_base | log2(info->gart_size), info->ioc_regs+IOC_PCOM);
+ writeq(info->gart_base | ilog2(info->gart_size), info->ioc_regs+IOC_PCOM);
readq(info->ioc_regs+IOC_PCOM); /* flush */
}
--- linux-2.6.22-rc6-mm1.orig/fs/ext4/super.c
+++ linux-2.6.22-rc6-mm1/fs/ext4/super.c
@@ -1433,8 +1433,6 @@ static void ext4_orphan_cleanup (struct
sb->s_flags = s_flags; /* Restore MS_RDONLY status */
}
-#define log2(n) ffz(~(n))
-
/*
* Maximal file size. There is a direct, and {,double-,triple-}indirect
* block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks.
@@ -1706,8 +1704,8 @@ static int ext4_fill_super (struct super
sbi->s_desc_per_block = blocksize / EXT4_DESC_SIZE(sb);
sbi->s_sbh = bh;
sbi->s_mount_state = le16_to_cpu(es->s_state);
- sbi->s_addr_per_block_bits = log2(EXT4_ADDR_PER_BLOCK(sb));
- sbi->s_desc_per_block_bits = log2(EXT4_DESC_PER_BLOCK(sb));
+ sbi->s_addr_per_block_bits = ilog2(EXT4_ADDR_PER_BLOCK(sb));
+ sbi->s_desc_per_block_bits = ilog2(EXT4_DESC_PER_BLOCK(sb));
for (i=0; i < 4; i++)
sbi->s_hash_seed[i] = le32_to_cpu(es->s_hash_seed[i]);
sbi->s_def_hash_version = es->s_def_hash_version;
-
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