Libguestfs - Feb 2021 - [PATCH libnbd v3 2/3] copy: Preserve the host page cache when reading from local files.

When reading from a local file we can take advantage of the page cache
(ie. not having to read the file from disk if a copy is present in
memory), while at the same time not disturbing the state of the page
cache.  Disturbing the page cache can have bad consequences for other
processes running on the host since they will have their working set
evicted so it is something we should generally avoid.

This requires Linux APIs, using the technique described here:
https://insights.oetiker.ch/linux/fadvise/

This change only affects reads, since doing the same for writes is
even more complicated.

You can see the effect using the tools from
https://github.com/Feh/nocache

Before this change:

  $ cachestats /var/tmp/random
  pages in cache: 315648/8388608 (3.8%)  [filesize=33554432.0K, pagesize=4K]
  $ ./run time nbdcopy /var/tmp/random null:
  1.80user 23.70system 0:20.69elapsed 123%CPU (0avgtext+0avgdata
135084maxresident)k
  64585232inputs+0outputs (6major+8391948minor)pagefaults 0swaps
  $ cachestats /var/tmp/random
  pages in cache: 7734356/8388608 (92.2%)  [filesize=33554432.0K, pagesize=4K]

Notice that a large part of the file has been loaded into the page
cache after the run.

After this change:

  $ cachestats /var/tmp/random
  pages in cache: 315008/8388608 (3.8%)  [filesize=33554432.0K, pagesize=4K]
  $ time ./run nbdcopy /var/tmp/random null:
  real	0m21.602s
  user	0m1.944s
  sys	0m29.013s
  $ cachestats /var/tmp/random
  pages in cache: 315648/8388608 (3.8%)  [filesize=33554432.0K, pagesize=4K]

Notice there is only a small increase in the amount of file which is
cached, the elapsed time is about the same, but there is an increase
in system time (presumably the overhead of POSIX_FADV_DONTNEED).
---
 copy/file-ops.c | 130 +++++++++++++++++++++++++++++++++++++++++++++++-
 copy/main.c     |   2 +-
 copy/nbdcopy.h  |   2 +-
 3 files changed, 131 insertions(+), 3 deletions(-)

diff --git a/copy/file-ops.c b/copy/file-ops.c
index 73cbdcb..888a388 100644
--- a/copy/file-ops.c
+++ b/copy/file-ops.c
@@ -24,8 +24,10 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
+#include <limits.h>
 #include <sys/ioctl.h>
 #include <sys/types.h>
+#include <sys/mman.h>
 
 #include <pthread.h>
 
@@ -34,8 +36,27 @@
 #endif
 
 #include "isaligned.h"
+#include "rounding.h"
+
 #include "nbdcopy.h"
 
+/* If we are going to attempt page cache mapping which tries not to
+ * disturb the page cache when reading a file.  Only do this on Linux
+ * systems where we understand how the page cache behaves.  Since we
+ * need to mmap the whole file, also restrict this to 64 bit systems.
+ */
+#ifdef __linux__
+#ifdef __SIZEOF_POINTER__
+#if __SIZEOF_POINTER__ == 8
+#define PAGE_CACHE_MAPPING 1
+#endif
+#endif
+#endif
+
+#ifdef PAGE_CACHE_MAPPING
+DEFINE_VECTOR_TYPE (byte_vector, uint8_t)
+#endif
+
 static struct rw_ops file_ops;
 
 struct rw_file {
@@ -50,8 +71,96 @@ struct rw_file {
    * the working method.
    */
   bool can_punch_hole, can_zero_range, can_fallocate, can_zeroout;
+
+#ifdef PAGE_CACHE_MAPPING
+  byte_vector cached_pages;
+#endif
 };
 
+#ifdef PAGE_CACHE_MAPPING
+static long page_size;
+
+static void page_size_init (void) __attribute__((constructor));
+static void
+page_size_init (void)
+{
+  page_size = sysconf (_SC_PAGE_SIZE);
+}
+
+/* Load the page cache map for a particular file into
+ * rwf->cached_pages.  Only used when reading files.  This doesn't
+ * fail: if a system call fails then rwf->cached_pages.size will be
+ * zero which is handled in page_was_cached.
+ */
+static inline void
+page_cache_map (struct rw_file *rwf, int fd, int64_t size)
+{
+  void *ptr;
+
+  if (size == 0) return;
+
+  ptr = mmap (NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
+  if (ptr == (void *)-1) return;
+
+  const size_t veclen = ROUND_UP (size, page_size) / page_size;
+
+  if (byte_vector_reserve (&rwf->cached_pages, veclen) == -1)
+    goto err;
+  if (mincore (ptr, size, rwf->cached_pages.ptr) == -1)
+    goto err;
+
+  rwf->cached_pages.size = veclen;
+ err:
+  munmap (ptr, size);
+}
+
+/* Test if a single page of the file was cached before nbdcopy ran. */
+static inline bool
+page_was_cached (struct rw_file *rwf, uint64_t offset)
+{
+  uint64_t page = offset / page_size;
+  if (page < rwf->cached_pages.size)
+    return (rwf->cached_pages.ptr[page] & 1) != 0;
+  else
+    /* This path is taken if we didn't manage to map the input file
+     * for any reason.  In this case assume that pages were mapped so
+     * we will not evict them: essentially fall back to doing nothing.
+     */
+    return true;
+}
+
+/* Evict file contents from the page cache if they were not present in
+ * the page cache before.
+ */
+static inline void
+page_cache_evict (struct rw_file *rwf, uint64_t orig_offset, size_t orig_len)
+{
+  uint64_t offset, n;
+  size_t len;
+
+  if (rwf->cached_pages.size == 0) return;
+
+  /* Only bother with whole pages. */
+  offset = ROUND_UP (orig_offset, page_size);
+  len = orig_len - (offset - orig_offset);
+  len = ROUND_DOWN (len, page_size);
+
+  while (len > 0) {
+    n = page_size;
+    if (! page_was_cached (rwf, offset)) {
+      /* Try to evict runs of pages in one go. */
+      while (len-n > 0 && ! page_was_cached (rwf, offset+n))
+        n += page_size;
+
+      posix_fadvise (rwf->fd, offset, n, POSIX_FADV_DONTNEED);
+    }
+
+    offset += n;
+    len -= n;
+  }
+}
+#endif
+
 static bool
 seek_hole_supported (int fd)
 {
@@ -64,7 +173,8 @@ seek_hole_supported (int fd)
 }
 
 struct rw *
-file_create (const char *name, int fd, off_t st_size, bool is_block)
+file_create (const char *name, int fd,
+             off_t st_size, bool is_block, direction d)
 {
   struct rw_file *rwf = calloc (1, sizeof *rwf);
   if (rwf == NULL) { perror ("calloc"); exit (EXIT_FAILURE); }
@@ -123,6 +233,11 @@ file_create (const char *name, int fd, off_t st_size, bool
is_block)
   posix_fadvise (fd, 0, 0, POSIX_FADV_SEQUENTIAL);
 #endif
 
+#if PAGE_CACHE_MAPPING
+  if (d == READING)
+    page_cache_map (rwf, fd, rwf->rw.size);
+#endif
+
   return &rwf->rw;
 }
 
@@ -135,6 +250,11 @@ file_close (struct rw *rw)
     fprintf (stderr, "%s: close: %m\n", rw->name);
     exit (EXIT_FAILURE);
   }
+
+#ifdef PAGE_CACHE_MAPPING
+  byte_vector_reset (&rwf->cached_pages);
+#endif
+
   free (rw);
 }
 
@@ -211,6 +331,10 @@ file_synch_read (struct rw *rw,
                  void *data, size_t len, uint64_t offset)
 {
   struct rw_file *rwf = (struct rw_file *)rw;
+#ifdef PAGE_CACHE_MAPPING
+  const uint64_t orig_offset = offset;
+  const size_t orig_len = len;
+#endif
   size_t n = 0;
   ssize_t r;
 
@@ -229,6 +353,10 @@ file_synch_read (struct rw *rw,
     n += r;
   }
 
+#if PAGE_CACHE_MAPPING
+  page_cache_evict (rwf, orig_offset, orig_len);
+#endif
+
   return n;
 }
 
diff --git a/copy/main.c b/copy/main.c
index 3c574df..55c2b53 100644
--- a/copy/main.c
+++ b/copy/main.c
@@ -461,7 +461,7 @@ open_local (const char *filename, direction d)
     exit (EXIT_FAILURE);
   }
   if (S_ISBLK (stat.st_mode) || S_ISREG (stat.st_mode))
-    return file_create (filename, fd, stat.st_size, S_ISBLK (stat.st_mode));
+    return file_create (filename, fd, stat.st_size, S_ISBLK (stat.st_mode), d);
   else {
     /* Probably stdin/stdout, a pipe or a socket. */
     synchronous = true;        /* Force synchronous mode for pipes. */
diff --git a/copy/nbdcopy.h b/copy/nbdcopy.h
index 4496722..e4c3d4e 100644
--- a/copy/nbdcopy.h
+++ b/copy/nbdcopy.h
@@ -52,7 +52,7 @@ typedef enum { READING, WRITING } direction;
 
 /* Create subtypes. */
 extern struct rw *file_create (const char *name, int fd,
-                               off_t st_size, bool is_block);
+                               off_t st_size, bool is_block, direction d);
 extern struct rw *nbd_rw_create_uri (const char *name,
                                      const char *uri, direction d);
 extern struct rw *nbd_rw_create_subprocess (const char **argv, size_t argc,
-- 
2.29.0.rc2

On 2/25/21 11:34 AM, Richard W.M. Jones wrote:
> When reading from a local file we can take advantage of the page cache
> (ie. not having to read the file from disk if a copy is present in
> memory), while at the same time not disturbing the state of the page
> cache.  Disturbing the page cache can have bad consequences for other
> processes running on the host since they will have their working set
> evicted so it is something we should generally avoid.
Even more so when dealing with disk images that are likely to be larger
in size than available memory, which tends to be a rather common NBD
usage ;)
> 
> This requires Linux APIs, using the technique described here:
> https://insights.oetiker.ch/linux/fadvise/
> 
> This change only affects reads, since doing the same for writes is
> even more complicated.
> 
> 
> Notice there is only a small increase in the amount of file which is
> cached, the elapsed time is about the same, but there is an increase
> in system time (presumably the overhead of POSIX_FADV_DONTNEED).
The improved cache pressure at almost no elapsed time penalty is worth it.

>  
> +/* If we are going to attempt page cache mapping which tries not to
> + * disturb the page cache when reading a file.  Only do this on Linux
Works, but took me a couple reads to decide it wasn't a sentence
fragment.  Would it be any better as s/If/Whether/?
> + * systems where we understand how the page cache behaves.  Since we
> + * need to mmap the whole file, also restrict this to 64 bit systems.
> + */
> +#ifdef __linux__
> +#ifdef __SIZEOF_POINTER__
> +#if __SIZEOF_POINTER__ == 8
> +#define PAGE_CACHE_MAPPING 1
> +#endif
> +#endif
> +#endif
> +
>  
> +#ifdef PAGE_CACHE_MAPPING
> +static long page_size;
> +
> +static void page_size_init (void) __attribute__((constructor));
> +static void
> +page_size_init (void)
> +{
> +  page_size = sysconf (_SC_PAGE_SIZE);
Technically, sysconf() can fail; but if it fails on _SC_PAGE_SIZE,
you've got bigger problems ;)
> +/* Test if a single page of the file was cached before nbdcopy ran. */
> +static inline bool
> +page_was_cached (struct rw_file *rwf, uint64_t offset)
> +{
> +  uint64_t page = offset / page_size;
> +  if (page < rwf->cached_pages.size)
> +    return (rwf->cached_pages.ptr[page] & 1) != 0;
> +  else
> +    /* This path is taken if we didn't manage to map the input file
> +     * for any reason.  In this case assume that pages were mapped so
> +     * we will not evict them: essentially fall back to doing nothing.
> +     */
> +    return true;
Nice fallback.
> +}
> +
> +/* Evict file contents from the page cache if they were not present in
> + * the page cache before.
> + */
> +static inline void
> +page_cache_evict (struct rw_file *rwf, uint64_t orig_offset, size_t
orig_len)
> +{
> +  uint64_t offset, n;
> +  size_t len;
> +
> +  if (rwf->cached_pages.size == 0) return;
> +
> +  /* Only bother with whole pages. */
> +  offset = ROUND_UP (orig_offset, page_size);
> +  len = orig_len - (offset - orig_offset);
> +  len = ROUND_DOWN (len, page_size);
What assurance do we have that the rest of nbdcopy is favoring a stride
that is a multiple of page_size, so that we aren't stranding partial
pages everywhere?

Looks good.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org