[RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Dave Hansen]

I've been doing some testing involving large amounts of
page cache.  It's quite painful to get hundreds of GB
of page cache mapped in, especially when I am trying to
do it in parallel threads.  This is true even when the
page cache is already allocated and I only need to map
it in.  The test:

1. take 160 16MB files
2. clone 160 threads, mmap the 16MB files, and either
  a. walk through the file touching each page
  b. run MADV_POPULATE on the file
3. MADV_DONTNEED on the mmap()'d area

160 threads/processes:
	    faulting | MADV_POPULATE
  Threads:       698 |        102239 (146x speedup)
Proceeses:    154247 |        297518 (1.9x speedup)

single threaded:
	    faulting | MADV_POPULATE
                1908 |          3710 (1.9x speedup)

To fix the thread suckage, this patch just walks the
VMAs and maps all the pages in.  Since it does a
bunch of them in one go, it amortizes the cost of
acquiring the mmap_sem across all of those pages.

FAQ:

Why do threads suck so much?

	Bouncing the mmap_sem cacheline around, plus anything
	else that we write to during a fault.  We do one page,
	move the cachelines to another CPU, do one more page,
	etc...

Does MADV_DONTNEED work for this?

	No.  It brings the pages in to the page cache, but
	does not map them the way it is implemented at the
	moment.  I guess we'd be within our rights to make
	it behave like MADV_POPULATE if we want though.



---

 linux.git-davehans/include/uapi/asm-generic/mman-common.h |    1 
 linux.git-davehans/mm/madvise.c                           |   40 +++++++++++++-
 2 files changed, 40 insertions(+), 1 deletion(-)

diff -puN include/uapi/asm-generic/mman-common.h~madv_populate include/uapi/asm-generic/mman-common.h
--- linux.git/include/uapi/asm-generic/mman-common.h~madv_populate	2013-06-27 15:22:35.651854196 -0700
+++ linux.git-davehans/include/uapi/asm-generic/mman-common.h	2013-06-27 15:22:35.656854418 -0700
@@ -51,6 +51,7 @@
 #define MADV_DONTDUMP   16		/* Explicity exclude from the core dump,
 					   overrides the coredump filter bits */
 #define MADV_DODUMP	17		/* Clear the MADV_NODUMP flag */
+#define MADV_POPULATE	18		/* Fill in mapping like faults would */
 
 /* compatibility flags */
 #define MAP_FILE	0
diff -puN mm/madvise.c~madv_populate mm/madvise.c
--- linux.git/mm/madvise.c~madv_populate	2013-06-27 15:22:35.652854240 -0700
+++ linux.git-davehans/mm/madvise.c	2013-06-27 15:22:35.656854418 -0700
@@ -19,6 +19,7 @@
 #include <linux/blkdev.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
+#include "internal.h"
 
 /*
  * Any behaviour which results in changes to the vma->vm_flags needs to
@@ -31,6 +32,7 @@ static int madvise_need_mmap_write(int b
 	case MADV_REMOVE:
 	case MADV_WILLNEED:
 	case MADV_DONTNEED:
+	case MADV_POPULATE:
 		return 0;
 	default:
 		/* be safe, default to 1. list exceptions explicitly */
@@ -252,6 +254,39 @@ static long madvise_willneed(struct vm_a
 }
 
 /*
+ * Do not just populate the page cache (WILLNEED), also map the pages.
+ */
+static long madvise_populate(struct vm_area_struct * vma,
+			     struct vm_area_struct ** prev,
+			     unsigned long start, unsigned long end)
+{
+	struct file *file = vma->vm_file;
+	int locked = 1;
+	int ret;
+
+	if (file && file->f_mapping->a_ops->get_xip_mem) {
+		/* no bad return value, but ignore advice */
+		return 0;
+	}
+
+	ret = __mlock_vma_pages_range(vma, start, end, &locked);
+	/*
+	 * Make sure that out down_read() matches (read vs.
+	 * write) what we did in sys_madvise.
+	 */
+	BUG_ON(madvise_need_mmap_write(MADV_POPULATE));
+	if (!locked) {
+		down_read(&current->mm->mmap_sem);
+		/* tell sys_madvise we drop mmap_sem: */
+		*prev = NULL;
+	} else {
+		*prev = vma;
+	}
+
+	return ret;
+}
+
+/*
  * Application no longer needs these pages.  If the pages are dirty,
  * it's OK to just throw them away.  The app will be more careful about
  * data it wants to keep.  Be sure to free swap resources too.  The
@@ -378,6 +413,8 @@ madvise_vma(struct vm_area_struct *vma,
 		return madvise_remove(vma, prev, start, end);
 	case MADV_WILLNEED:
 		return madvise_willneed(vma, prev, start, end);
+	case MADV_POPULATE:
+		return madvise_populate(vma, prev, start, end);
 	case MADV_DONTNEED:
 		return madvise_dontneed(vma, prev, start, end);
 	default:
@@ -407,6 +444,7 @@ madvise_behavior_valid(int behavior)
 #endif
 	case MADV_DONTDUMP:
 	case MADV_DODUMP:
+	case MADV_POPULATE:
 		return 1;
 
 	default:
@@ -536,7 +574,7 @@ SYSCALL_DEFINE3(madvise, unsigned long,
 			goto out;
 		if (prev)
 			vma = prev->vm_next;
-		else	/* madvise_remove dropped mmap_sem */
+		else	/* madvise_remove/populate dropped mmap_sem */
 			vma = find_vma(current->mm, start);
 	}
 out:
_

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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Zheng Liu]

Hi Dave,

On Thu, Jun 27, 2013 at 04:16:05PM -0700, Dave Hansen wrote:
> 
> I've been doing some testing involving large amounts of
> page cache.  It's quite painful to get hundreds of GB
> of page cache mapped in, especially when I am trying to
> do it in parallel threads.  This is true even when the
> page cache is already allocated and I only need to map
> it in.  The test:
> 
> 1. take 160 16MB files
> 2. clone 160 threads, mmap the 16MB files, and either
>   a. walk through the file touching each page

Why not change MAP_POPULATE flag in mmap(2)?  Now it is only for private
mappings.  But maybe we could let it support shared mapping.

Regards,
                                                - Zheng

>   b. run MADV_POPULATE on the file
> 3. MADV_DONTNEED on the mmap()'d area
> 
> 160 threads/processes:
> 	    faulting | MADV_POPULATE
>   Threads:       698 |        102239 (146x speedup)
> Proceeses:    154247 |        297518 (1.9x speedup)
> 
> single threaded:
> 	    faulting | MADV_POPULATE
>                 1908 |          3710 (1.9x speedup)
> 
> To fix the thread suckage, this patch just walks the
> VMAs and maps all the pages in.  Since it does a
> bunch of them in one go, it amortizes the cost of
> acquiring the mmap_sem across all of those pages.
> 
> FAQ:
> 
> Why do threads suck so much?
> 
> 	Bouncing the mmap_sem cacheline around, plus anything
> 	else that we write to during a fault.  We do one page,
> 	move the cachelines to another CPU, do one more page,
> 	etc...
> 
> Does MADV_DONTNEED work for this?
> 
> 	No.  It brings the pages in to the page cache, but
> 	does not map them the way it is implemented at the
> 	moment.  I guess we'd be within our rights to make
> 	it behave like MADV_POPULATE if we want though.
> 
> 
> 
> ---
> 
>  linux.git-davehans/include/uapi/asm-generic/mman-common.h |    1 
>  linux.git-davehans/mm/madvise.c                           |   40 +++++++++++++-
>  2 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff -puN include/uapi/asm-generic/mman-common.h~madv_populate include/uapi/asm-generic/mman-common.h
> --- linux.git/include/uapi/asm-generic/mman-common.h~madv_populate	2013-06-27 15:22:35.651854196 -0700
> +++ linux.git-davehans/include/uapi/asm-generic/mman-common.h	2013-06-27 15:22:35.656854418 -0700
> @@ -51,6 +51,7 @@
>  #define MADV_DONTDUMP   16		/* Explicity exclude from the core dump,
>  					   overrides the coredump filter bits */
>  #define MADV_DODUMP	17		/* Clear the MADV_NODUMP flag */
> +#define MADV_POPULATE	18		/* Fill in mapping like faults would */
>  
>  /* compatibility flags */
>  #define MAP_FILE	0
> diff -puN mm/madvise.c~madv_populate mm/madvise.c
> --- linux.git/mm/madvise.c~madv_populate	2013-06-27 15:22:35.652854240 -0700
> +++ linux.git-davehans/mm/madvise.c	2013-06-27 15:22:35.656854418 -0700
> @@ -19,6 +19,7 @@
>  #include <linux/blkdev.h>
>  #include <linux/swap.h>
>  #include <linux/swapops.h>
> +#include "internal.h"
>  
>  /*
>   * Any behaviour which results in changes to the vma->vm_flags needs to
> @@ -31,6 +32,7 @@ static int madvise_need_mmap_write(int b
>  	case MADV_REMOVE:
>  	case MADV_WILLNEED:
>  	case MADV_DONTNEED:
> +	case MADV_POPULATE:
>  		return 0;
>  	default:
>  		/* be safe, default to 1. list exceptions explicitly */
> @@ -252,6 +254,39 @@ static long madvise_willneed(struct vm_a
>  }
>  
>  /*
> + * Do not just populate the page cache (WILLNEED), also map the pages.
> + */
> +static long madvise_populate(struct vm_area_struct * vma,
> +			     struct vm_area_struct ** prev,
> +			     unsigned long start, unsigned long end)
> +{
> +	struct file *file = vma->vm_file;
> +	int locked = 1;
> +	int ret;
> +
> +	if (file && file->f_mapping->a_ops->get_xip_mem) {
> +		/* no bad return value, but ignore advice */
> +		return 0;
> +	}
> +
> +	ret = __mlock_vma_pages_range(vma, start, end, &locked);
> +	/*
> +	 * Make sure that out down_read() matches (read vs.
> +	 * write) what we did in sys_madvise.
> +	 */
> +	BUG_ON(madvise_need_mmap_write(MADV_POPULATE));
> +	if (!locked) {
> +		down_read(&current->mm->mmap_sem);
> +		/* tell sys_madvise we drop mmap_sem: */
> +		*prev = NULL;
> +	} else {
> +		*prev = vma;
> +	}
> +
> +	return ret;
> +}
> +
> +/*
>   * Application no longer needs these pages.  If the pages are dirty,
>   * it's OK to just throw them away.  The app will be more careful about
>   * data it wants to keep.  Be sure to free swap resources too.  The
> @@ -378,6 +413,8 @@ madvise_vma(struct vm_area_struct *vma,
>  		return madvise_remove(vma, prev, start, end);
>  	case MADV_WILLNEED:
>  		return madvise_willneed(vma, prev, start, end);
> +	case MADV_POPULATE:
> +		return madvise_populate(vma, prev, start, end);
>  	case MADV_DONTNEED:
>  		return madvise_dontneed(vma, prev, start, end);
>  	default:
> @@ -407,6 +444,7 @@ madvise_behavior_valid(int behavior)
>  #endif
>  	case MADV_DONTDUMP:
>  	case MADV_DODUMP:
> +	case MADV_POPULATE:
>  		return 1;
>  
>  	default:
> @@ -536,7 +574,7 @@ SYSCALL_DEFINE3(madvise, unsigned long,
>  			goto out;
>  		if (prev)
>  			vma = prev->vm_next;
> -		else	/* madvise_remove dropped mmap_sem */
> +		else	/* madvise_remove/populate dropped mmap_sem */
>  			vma = find_vma(current->mm, start);
>  	}
>  out:
> _
> 
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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Dave Hansen]

On 06/27/2013 10:47 PM, Zheng Liu wrote:
>> I've been doing some testing involving large amounts of
>> page cache.  It's quite painful to get hundreds of GB
>> of page cache mapped in, especially when I am trying to
>> do it in parallel threads.  This is true even when the
>> page cache is already allocated and I only need to map
>> it in.  The test:
>>
>> 1. take 160 16MB files
>> 2. clone 160 threads, mmap the 16MB files, and either
>>   a. walk through the file touching each page
> 
> Why not change MAP_POPULATE flag in mmap(2)?  Now it is only for private
> mappings.  But maybe we could let it support shared mapping.

Adding that support to mmap() will certainly _help_ some folks.  But,
anything that mmap()s something is taking mmap_sem for write.  That
means that threaded apps doing mmap()/munmap() frequently are _not_
scalable.

IOW, a process needing to do a bunch of MAP_POPULATEs isn't
parallelizable, but one using this mechanism would be.

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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Zheng Liu]

On 06/28/2013 11:48 PM, Dave Hansen wrote:
> On 06/27/2013 10:47 PM, Zheng Liu wrote:
>>> I've been doing some testing involving large amounts of
>>> page cache.  It's quite painful to get hundreds of GB
>>> of page cache mapped in, especially when I am trying to
>>> do it in parallel threads.  This is true even when the
>>> page cache is already allocated and I only need to map
>>> it in.  The test:
>>>
>>> 1. take 160 16MB files
>>> 2. clone 160 threads, mmap the 16MB files, and either
>>>   a. walk through the file touching each page
>>
>> Why not change MAP_POPULATE flag in mmap(2)?  Now it is only for private
>> mappings.  But maybe we could let it support shared mapping.
> 
> Adding that support to mmap() will certainly _help_ some folks.  But,
> anything that mmap()s something is taking mmap_sem for write.  That
> means that threaded apps doing mmap()/munmap() frequently are _not_
> scalable.
> 
> IOW, a process needing to do a bunch of MAP_POPULATEs isn't
> parallelizable, but one using this mechanism would be.

I look at the code, and it seems that we will handle MAP_POPULATE flag
after we release mmap_sem locking in vm_mmap_pgoff():

                down_write(&mm->mmap_sem);
                ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
                                    &populate);
                up_write(&mm->mmap_sem);
                if (populate)
                        mm_populate(ret, populate);

Am I missing something?

Regards,
						- Zheng

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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Dave Hansen]

On 06/28/2013 07:20 PM, Zheng Liu wrote:
>> > IOW, a process needing to do a bunch of MAP_POPULATEs isn't
>> > parallelizable, but one using this mechanism would be.
> I look at the code, and it seems that we will handle MAP_POPULATE flag
> after we release mmap_sem locking in vm_mmap_pgoff():
> 
>                 down_write(&mm->mmap_sem);
>                 ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
>                                     &populate);
>                 up_write(&mm->mmap_sem);
>                 if (populate)
>                         mm_populate(ret, populate);
> 
> Am I missing something?

I went and did my same test using mmap(MAP_POPULATE)/munmap() pair
versus using MADV_POPULATE in 160 threads in parallel.

MADV_POPULATE was about 10x faster in the threaded configuration.

With MADV_POPULATE, the biggest cost is shipping the mmap_sem cacheline
around so that we can write the reader count update in to it.  With
mmap(), there is a lot of _contention_ on that lock which is much, much
more expensive than simply bouncing a cacheline around.

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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Zheng Liu]

On Mon, Jul 01, 2013 at 09:16:46AM -0700, Dave Hansen wrote:
> On 06/28/2013 07:20 PM, Zheng Liu wrote:
> >> > IOW, a process needing to do a bunch of MAP_POPULATEs isn't
> >> > parallelizable, but one using this mechanism would be.
> > I look at the code, and it seems that we will handle MAP_POPULATE flag
> > after we release mmap_sem locking in vm_mmap_pgoff():
> > 
> >                 down_write(&mm->mmap_sem);
> >                 ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
> >                                     &populate);
> >                 up_write(&mm->mmap_sem);
> >                 if (populate)
> >                         mm_populate(ret, populate);
> > 
> > Am I missing something?
> 
> I went and did my same test using mmap(MAP_POPULATE)/munmap() pair
> versus using MADV_POPULATE in 160 threads in parallel.
> 
> MADV_POPULATE was about 10x faster in the threaded configuration.
> 
> With MADV_POPULATE, the biggest cost is shipping the mmap_sem cacheline
> around so that we can write the reader count update in to it.  With
> mmap(), there is a lot of _contention_ on that lock which is much, much
> more expensive than simply bouncing a cacheline around.

Thanks for your explanation.

FWIW, it would be great if we can let MAP_POPULATE flag support shared
mappings because in our product system there has a lot of applications
that uses mmap(2) and then pre-faults this mapping.  Currently these
applications need to pre-fault the mapping manually.

Regards,
                                                - Zheng

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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Dave Hansen]

On 07/01/2013 07:37 PM, Zheng Liu wrote:
> FWIW, it would be great if we can let MAP_POPULATE flag support shared
> mappings because in our product system there has a lot of applications
> that uses mmap(2) and then pre-faults this mapping.  Currently these
> applications need to pre-fault the mapping manually.

Are you sure it doesn't?  From a cursory look at the code, it looked to
me like it would populate anonymous and file-backed, but I didn't
double-check experimentally.

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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Zheng Liu]

On Mon, Jul 01, 2013 at 09:43:29PM -0700, Dave Hansen wrote:
> On 07/01/2013 07:37 PM, Zheng Liu wrote:
> > FWIW, it would be great if we can let MAP_POPULATE flag support shared
> > mappings because in our product system there has a lot of applications
> > that uses mmap(2) and then pre-faults this mapping.  Currently these
> > applications need to pre-fault the mapping manually.
> 
> Are you sure it doesn't?  From a cursory look at the code, it looked to
> me like it would populate anonymous and file-backed, but I didn't
> double-check experimentally.

Thanks for pointing it out. I write a program to test this issue, and it
seems to me that it can populate a shared mapping.  But in manpage it
describes as below:

MAP_POPULATE (since Linux 2.5.46)
    Populate (prefault) page tables for a mapping.  For a file mapping,
    this causes read-ahead on the file.  Later accesses to the mapping
    will not be blocked by page faults.  MAP_POPULATE is only supported
    for private mappings since Linux 2.6.23.

This page is part of release 3.24 of the Linux man-pages project.  I am
not sure whether it has been updated or not.

Regards,
                                                - Zheng

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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Sam Ben]

On 07/02/2013 10:37 AM, Zheng Liu wrote:
> On Mon, Jul 01, 2013 at 09:16:46AM -0700, Dave Hansen wrote:
>> On 06/28/2013 07:20 PM, Zheng Liu wrote:
>>>>> IOW, a process needing to do a bunch of MAP_POPULATEs isn't
>>>>> parallelizable, but one using this mechanism would be.
>>> I look at the code, and it seems that we will handle MAP_POPULATE flag
>>> after we release mmap_sem locking in vm_mmap_pgoff():
>>>
>>>                  down_write(&mm->mmap_sem);
>>>                  ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
>>>                                      &populate);
>>>                  up_write(&mm->mmap_sem);
>>>                  if (populate)
>>>                          mm_populate(ret, populate);
>>>
>>> Am I missing something?
>> I went and did my same test using mmap(MAP_POPULATE)/munmap() pair
>> versus using MADV_POPULATE in 160 threads in parallel.
>>
>> MADV_POPULATE was about 10x faster in the threaded configuration.
>>
>> With MADV_POPULATE, the biggest cost is shipping the mmap_sem cacheline
>> around so that we can write the reader count update in to it.  With
>> mmap(), there is a lot of _contention_ on that lock which is much, much
>> more expensive than simply bouncing a cacheline around.
> Thanks for your explanation.
>
> FWIW, it would be great if we can let MAP_POPULATE flag support shared
> mappings because in our product system there has a lot of applications
> that uses mmap(2) and then pre-faults this mapping.  Currently these
> applications need to pre-fault the mapping manually.

How do you pre-fault the mapping manually in your product system? By 
walking through the file touching each page?

>
> Regards,
>                                                  - Zheng
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Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Zheng Liu]

On Sun, Jul 14, 2013 at 11:12:58AM +0800, Sam Ben wrote:
> On 07/02/2013 10:37 AM, Zheng Liu wrote:
> >On Mon, Jul 01, 2013 at 09:16:46AM -0700, Dave Hansen wrote:
> >>On 06/28/2013 07:20 PM, Zheng Liu wrote:
> >>>>>IOW, a process needing to do a bunch of MAP_POPULATEs isn't
> >>>>>parallelizable, but one using this mechanism would be.
> >>>I look at the code, and it seems that we will handle MAP_POPULATE flag
> >>>after we release mmap_sem locking in vm_mmap_pgoff():
> >>>
> >>>                 down_write(&mm->mmap_sem);
> >>>                 ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
> >>>                                     &populate);
> >>>                 up_write(&mm->mmap_sem);
> >>>                 if (populate)
> >>>                         mm_populate(ret, populate);
> >>>
> >>>Am I missing something?
> >>I went and did my same test using mmap(MAP_POPULATE)/munmap() pair
> >>versus using MADV_POPULATE in 160 threads in parallel.
> >>
> >>MADV_POPULATE was about 10x faster in the threaded configuration.
> >>
> >>With MADV_POPULATE, the biggest cost is shipping the mmap_sem cacheline
> >>around so that we can write the reader count update in to it.  With
> >>mmap(), there is a lot of _contention_ on that lock which is much, much
> >>more expensive than simply bouncing a cacheline around.
> >Thanks for your explanation.
> >
> >FWIW, it would be great if we can let MAP_POPULATE flag support shared
> >mappings because in our product system there has a lot of applications
> >that uses mmap(2) and then pre-faults this mapping.  Currently these
> >applications need to pre-fault the mapping manually.
> 
> How do you pre-fault the mapping manually in your product system? By
> walking through the file touching each page?

Yes, in our product system most applications do like this.

Regards,
                                                - Zheng

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