[Qemu-devel] [PATCH v4 0/3] Clean unused entries in the qcow2 L2/refcount cache

[Qemu-devel] [PATCH v4 0/3] Clean unused entries in the qcow2 L2/refcount cache

[Alberto Garcia]

v4:
- Revert the 'cache-clean-interval' change. This should probably go
  into a new BlockDeviceInfoSpecific struct (along with other
  settings), but is out of the scope for this series.

v3: https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg05473.html
- Add 'cache-clean-interval' field to ImageInfoSpecificQCow2.

v2: https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg05316.html
- Clarify that the block-commit mentioned in the first patch refers to
  the HMP commit command.
- Check the value of cache_clean_interval and cast it accordingly to
  prevent it from overflowing.

v1: https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg03510.html

Regards,

Berto

Alberto Garcia (3):
  qcow2: mark the memory as no longer needed after qcow2_cache_empty()
  qcow2: add option to clean unused cache entries after some time
  qcow2: reorder fields in Qcow2CachedTable to reduce padding

 block/qcow2-cache.c  | 57 +++++++++++++++++++++++++++++++++++++++++++++-
 block/qcow2.c        | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.h        |  4 ++++
 qapi/block-core.json |  6 ++++-
 4 files changed, 129 insertions(+), 2 deletions(-)

-- 
2.1.4

[Qemu-devel] [PATCH 1/3] qcow2: mark the memory as no longer needed after qcow2_cache_empty()

[Alberto Garcia]

After having emptied the cache, the data in the cache tables is no
longer useful, so we can tell the kernel that we are done with it. In
Linux this frees the resources associated with it.

The effect of this can be seen in the HMP commit operation: it moves
data from the top to the base image (and fills both caches), then it
empties the top image. At this point the data in that cache is no
longer needed so it's just wasting memory.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cache.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index ed92a09..ed14a92 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -22,8 +22,10 @@
  * THE SOFTWARE.
  */
 
+#include <sys/mman.h>
 #include "block/block_int.h"
 #include "qemu-common.h"
+#include "qemu/osdep.h"
 #include "qcow2.h"
 #include "trace.h"
 
@@ -60,6 +62,22 @@ static inline int qcow2_cache_get_table_idx(BlockDriverState *bs,
     return idx;
 }
 
+static void qcow2_cache_table_release(BlockDriverState *bs, Qcow2Cache *c,
+                                      int i, int num_tables)
+{
+#if QEMU_MADV_DONTNEED != QEMU_MADV_INVALID
+    BDRVQcowState *s = bs->opaque;
+    void *t = qcow2_cache_get_table_addr(bs, c, i);
+    long align = sysconf(_SC_PAGESIZE);
+    size_t mem_size = (size_t) s->cluster_size * num_tables;
+    size_t offset = QEMU_ALIGN_UP((uintptr_t) t, align) - (uintptr_t) t;
+    size_t length = QEMU_ALIGN_DOWN(mem_size - offset, align);
+    if (length > 0) {
+        qemu_madvise((uint8_t *) t + offset, length, QEMU_MADV_DONTNEED);
+    }
+#endif
+}
+
 Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables)
 {
     BDRVQcowState *s = bs->opaque;
@@ -237,6 +255,8 @@ int qcow2_cache_empty(BlockDriverState *bs, Qcow2Cache *c)
         c->entries[i].lru_counter = 0;
     }
 
+    qcow2_cache_table_release(bs, c, 0, c->size);
+
     c->lru_counter = 0;
 
     return 0;
-- 
2.1.4

Re: [Qemu-devel] [PATCH 1/3] qcow2: mark the memory as no longer needed after qcow2_cache_empty()

[Kevin Wolf]

Am 29.05.2015 um 11:24 hat Alberto Garcia geschrieben:
> After having emptied the cache, the data in the cache tables is no
> longer useful, so we can tell the kernel that we are done with it. In
> Linux this frees the resources associated with it.
> 
> The effect of this can be seen in the HMP commit operation: it moves
> data from the top to the base image (and fills both caches), then it
> empties the top image. At this point the data in that cache is no
> longer needed so it's just wasting memory.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-cache.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
> index ed92a09..ed14a92 100644
> --- a/block/qcow2-cache.c
> +++ b/block/qcow2-cache.c
> @@ -22,8 +22,10 @@
>   * THE SOFTWARE.
>   */
>  
> +#include <sys/mman.h>
>  #include "block/block_int.h"
>  #include "qemu-common.h"
> +#include "qemu/osdep.h"
>  #include "qcow2.h"
>  #include "trace.h"

This breaks the mingw build:

/mnt/qemu/block/qcow2-cache.c:25:22: fatal error: sys/mman.h: No such file or directory
 #include <sys/mman.h>

> @@ -60,6 +62,22 @@ static inline int qcow2_cache_get_table_idx(BlockDriverState *bs,
>      return idx;
>  }
>  
> +static void qcow2_cache_table_release(BlockDriverState *bs, Qcow2Cache *c,
> +                                      int i, int num_tables)
> +{
> +#if QEMU_MADV_DONTNEED != QEMU_MADV_INVALID
> +    BDRVQcowState *s = bs->opaque;
> +    void *t = qcow2_cache_get_table_addr(bs, c, i);
> +    long align = sysconf(_SC_PAGESIZE);

It seems that getpagesize() is usually used in qemu.

> +    size_t mem_size = (size_t) s->cluster_size * num_tables;
> +    size_t offset = QEMU_ALIGN_UP((uintptr_t) t, align) - (uintptr_t) t;
> +    size_t length = QEMU_ALIGN_DOWN(mem_size - offset, align);

Instead of all the aligning here, shouldn't we just make sure that the
tables are created with the right alignment?

> +    if (length > 0) {
> +        qemu_madvise((uint8_t *) t + offset, length, QEMU_MADV_DONTNEED);
> +    }
> +#endif
> +}

Kevin

Re: [Qemu-devel] [PATCH 1/3] qcow2: mark the memory as no longer needed after qcow2_cache_empty()

[Alberto Garcia]

On Tue 02 Jun 2015 12:05:59 PM CEST, Kevin Wolf <kwolf@redhat.com> wrote:

>> +#include <sys/mman.h>
>>  #include "block/block_int.h"
>>  #include "qemu-common.h"
>> +#include "qemu/osdep.h"
>>  #include "qcow2.h"
>>  #include "trace.h"
>
> This breaks the mingw build:
>
> /mnt/qemu/block/qcow2-cache.c:25:22: fatal error: sys/mman.h: No such file or directory
>  #include <sys/mman.h>

Ok, I can use #if defined(CONFIG_MADVISE) || defined(CONFIG_POSIX_MADVISE)
as it's done in util/osdep.c for the same header.

>> +#if QEMU_MADV_DONTNEED != QEMU_MADV_INVALID
>> +    BDRVQcowState *s = bs->opaque;
>> +    void *t = qcow2_cache_get_table_addr(bs, c, i);
>> +    long align = sysconf(_SC_PAGESIZE);
>
> It seems that getpagesize() is usually used in qemu.

The getpagesize() manual page actually recommends using sysconf() for
portability reasons. But other than that I don't have a problem with
getpagesize() if it's the preferred choice in QEMU.

>> +    size_t mem_size = (size_t) s->cluster_size * num_tables;
>> +    size_t offset = QEMU_ALIGN_UP((uintptr_t) t, align) - (uintptr_t) t;
>> +    size_t length = QEMU_ALIGN_DOWN(mem_size - offset, align);
>
> Instead of all the aligning here, shouldn't we just make sure that the
> tables are created with the right alignment?

The tables should have the right alignment and their size should be a
multiple of the page size. The latter condition is not necessarily true
(a cluster can be smaller than one page) so I'd keep that code in any
case.

Berto

Re: [Qemu-devel] [PATCH 1/3] qcow2: mark the memory as no longer needed after qcow2_cache_empty()

[Kevin Wolf]

Am 02.06.2015 um 12:50 hat Alberto Garcia geschrieben:
> On Tue 02 Jun 2015 12:05:59 PM CEST, Kevin Wolf <kwolf@redhat.com> wrote:
> 
> >> +#include <sys/mman.h>
> >>  #include "block/block_int.h"
> >>  #include "qemu-common.h"
> >> +#include "qemu/osdep.h"
> >>  #include "qcow2.h"
> >>  #include "trace.h"
> >
> > This breaks the mingw build:
> >
> > /mnt/qemu/block/qcow2-cache.c:25:22: fatal error: sys/mman.h: No such file or directory
> >  #include <sys/mman.h>
> 
> Ok, I can use #if defined(CONFIG_MADVISE) || defined(CONFIG_POSIX_MADVISE)
> as it's done in util/osdep.c for the same header.
> 
> >> +#if QEMU_MADV_DONTNEED != QEMU_MADV_INVALID
> >> +    BDRVQcowState *s = bs->opaque;
> >> +    void *t = qcow2_cache_get_table_addr(bs, c, i);
> >> +    long align = sysconf(_SC_PAGESIZE);
> >
> > It seems that getpagesize() is usually used in qemu.
> 
> The getpagesize() manual page actually recommends using sysconf() for
> portability reasons. But other than that I don't have a problem with
> getpagesize() if it's the preferred choice in QEMU.

There is a Windows implementation for getpagesize() in qemu, so I guess
it's actually more portable in our context.

Now, as you'll #ifdef this code out for Windows, that probably doesn't
matter, so it's just about consistency.

> >> +    size_t mem_size = (size_t) s->cluster_size * num_tables;
> >> +    size_t offset = QEMU_ALIGN_UP((uintptr_t) t, align) - (uintptr_t) t;
> >> +    size_t length = QEMU_ALIGN_DOWN(mem_size - offset, align);
> >
> > Instead of all the aligning here, shouldn't we just make sure that the
> > tables are created with the right alignment?
> 
> The tables should have the right alignment and their size should be a
> multiple of the page size. The latter condition is not necessarily true
> (a cluster can be smaller than one page) so I'd keep that code in any
> case.

We could adjust the size to MAX(size, pagesize), but perhaps that wastes
a bit too much memory indeed.

Kevin

Re: [Qemu-devel] [PATCH 1/3] qcow2: mark the memory as no longer needed after qcow2_cache_empty()

[Alberto Garcia]

On Tue 02 Jun 2015 12:56:10 PM CEST, Kevin Wolf wrote:

>> > It seems that getpagesize() is usually used in qemu.
>> 
>> The getpagesize() manual page actually recommends using sysconf() for
>> portability reasons. But other than that I don't have a problem with
>> getpagesize() if it's the preferred choice in QEMU.
>
> There is a Windows implementation for getpagesize() in qemu, so I
> guess it's actually more portable in our context.

Ok, I'll change that.

> Now, as you'll #ifdef this code out for Windows, that probably doesn't
> matter, so it's just about consistency.

There's actually no need to #ifdef that code out, I only do it because I
know in advance that it's going to be a no-op, but qemu_madvise() itself
doesn't break or anything if the operation is not supported.

Berto

[Qemu-devel] [PATCH 2/3] qcow2: add option to clean unused cache entries after some time

[Alberto Garcia]

This adds a new 'cache-clean-interval' option that cleans all qcow2
cache entries that haven't been used in a certain interval, given in
seconds.

This allows setting a large L2 cache size so it can handle scenarios
with lots of I/O and at the same time use little memory during periods
of inactivity.

This feature currently relies on MADV_DONTNEED to free that memory, so
it is not useful in systems that don't follow that behavior.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cache.c  | 35 ++++++++++++++++++++++++++++
 block/qcow2.c        | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.h        |  4 ++++
 qapi/block-core.json |  6 ++++-
 4 files changed, 108 insertions(+), 1 deletion(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index ed14a92..a215f5b 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -43,6 +43,7 @@ struct Qcow2Cache {
     bool                    depends_on_flush;
     void                   *table_array;
     uint64_t                lru_counter;
+    uint64_t                cache_clean_lru_counter;
 };
 
 static inline void *qcow2_cache_get_table_addr(BlockDriverState *bs,
@@ -78,6 +79,40 @@ static void qcow2_cache_table_release(BlockDriverState *bs, Qcow2Cache *c,
 #endif
 }
 
+static inline bool can_clean_entry(Qcow2Cache *c, int i)
+{
+    Qcow2CachedTable *t = &c->entries[i];
+    return t->ref == 0 && !t->dirty && t->offset != 0 &&
+        t->lru_counter <= c->cache_clean_lru_counter;
+}
+
+void qcow2_cache_clean_unused(BlockDriverState *bs, Qcow2Cache *c)
+{
+    int i = 0;
+    while (i < c->size) {
+        int to_clean = 0;
+
+        /* Skip the entries that we don't need to clean */
+        while (i < c->size && !can_clean_entry(c, i)) {
+            i++;
+        }
+
+        /* And count how many we can clean in a row */
+        while (i < c->size && can_clean_entry(c, i)) {
+            c->entries[i].offset = 0;
+            c->entries[i].lru_counter = 0;
+            i++;
+            to_clean++;
+        }
+
+        if (to_clean > 0) {
+            qcow2_cache_table_release(bs, c, i - to_clean, to_clean);
+        }
+    }
+
+    c->cache_clean_lru_counter = c->lru_counter;
+}
+
 Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables)
 {
     BDRVQcowState *s = bs->opaque;
diff --git a/block/qcow2.c b/block/qcow2.c
index f7b4cc6..8c565e9 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -468,6 +468,11 @@ static QemuOptsList qcow2_runtime_opts = {
             .type = QEMU_OPT_SIZE,
             .help = "Maximum refcount block cache size",
         },
+        {
+            .name = QCOW2_OPT_CACHE_CLEAN_INTERVAL,
+            .type = QEMU_OPT_NUMBER,
+            .help = "Clean unused cache entries after this time (in seconds)",
+        },
         { /* end of list */ }
     },
 };
@@ -483,6 +488,49 @@ static const char *overlap_bool_option_names[QCOW2_OL_MAX_BITNR] = {
     [QCOW2_OL_INACTIVE_L2_BITNR]    = QCOW2_OPT_OVERLAP_INACTIVE_L2,
 };
 
+static void cache_clean_timer_cb(void *opaque)
+{
+    BlockDriverState *bs = opaque;
+    BDRVQcowState *s = bs->opaque;
+    qcow2_cache_clean_unused(bs, s->l2_table_cache);
+    qcow2_cache_clean_unused(bs, s->refcount_block_cache);
+    timer_mod(s->cache_clean_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
+              (int64_t) s->cache_clean_interval * 1000);
+}
+
+static void cache_clean_timer_init(BlockDriverState *bs, AioContext *context)
+{
+    BDRVQcowState *s = bs->opaque;
+    if (s->cache_clean_interval > 0) {
+        s->cache_clean_timer = aio_timer_new(context, QEMU_CLOCK_VIRTUAL,
+                                             SCALE_MS, cache_clean_timer_cb,
+                                             bs);
+        timer_mod(s->cache_clean_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
+                  (int64_t) s->cache_clean_interval * 1000);
+    }
+}
+
+static void cache_clean_timer_del(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    if (s->cache_clean_timer) {
+        timer_del(s->cache_clean_timer);
+        timer_free(s->cache_clean_timer);
+        s->cache_clean_timer = NULL;
+    }
+}
+
+static void qcow2_detach_aio_context(BlockDriverState *bs)
+{
+    cache_clean_timer_del(bs);
+}
+
+static void qcow2_attach_aio_context(BlockDriverState *bs,
+                                     AioContext *new_context)
+{
+    cache_clean_timer_init(bs, new_context);
+}
+
 static void read_cache_sizes(QemuOpts *opts, uint64_t *l2_cache_size,
                              uint64_t *refcount_cache_size, Error **errp)
 {
@@ -552,6 +600,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     const char *opt_overlap_check, *opt_overlap_check_template;
     int overlap_check_template = 0;
     uint64_t l2_cache_size, refcount_cache_size;
+    uint64_t cache_clean_interval;
 
     ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
     if (ret < 0) {
@@ -839,6 +888,16 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
         goto fail;
     }
 
+    cache_clean_interval =
+        qemu_opt_get_number(opts, QCOW2_OPT_CACHE_CLEAN_INTERVAL, 0);
+    if (cache_clean_interval > UINT_MAX) {
+        error_setg(errp, "Cache clean interval too big");
+        ret = -EINVAL;
+        goto fail;
+    }
+    s->cache_clean_interval = cache_clean_interval;
+    cache_clean_timer_init(bs, bdrv_get_aio_context(bs));
+
     s->cluster_cache = g_malloc(s->cluster_size);
     /* one more sector for decompressed data alignment */
     s->cluster_data = qemu_try_blockalign(bs->file, QCOW_MAX_CRYPT_CLUSTERS
@@ -1004,6 +1063,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     qemu_vfree(s->l1_table);
     /* else pre-write overlap checks in cache_destroy may crash */
     s->l1_table = NULL;
+    cache_clean_timer_del(bs);
     if (s->l2_table_cache) {
         qcow2_cache_destroy(bs, s->l2_table_cache);
     }
@@ -1458,6 +1518,7 @@ static void qcow2_close(BlockDriverState *bs)
         }
     }
 
+    cache_clean_timer_del(bs);
     qcow2_cache_destroy(bs, s->l2_table_cache);
     qcow2_cache_destroy(bs, s->refcount_block_cache);
 
@@ -2970,6 +3031,9 @@ BlockDriver bdrv_qcow2 = {
     .create_opts         = &qcow2_create_opts,
     .bdrv_check          = qcow2_check,
     .bdrv_amend_options  = qcow2_amend_options,
+
+    .bdrv_detach_aio_context  = qcow2_detach_aio_context,
+    .bdrv_attach_aio_context  = qcow2_attach_aio_context,
 };
 
 static void bdrv_qcow2_init(void)
diff --git a/block/qcow2.h b/block/qcow2.h
index 0076512..2c45eb2 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -93,6 +93,7 @@
 #define QCOW2_OPT_CACHE_SIZE "cache-size"
 #define QCOW2_OPT_L2_CACHE_SIZE "l2-cache-size"
 #define QCOW2_OPT_REFCOUNT_CACHE_SIZE "refcount-cache-size"
+#define QCOW2_OPT_CACHE_CLEAN_INTERVAL "cache-clean-interval"
 
 typedef struct QCowHeader {
     uint32_t magic;
@@ -236,6 +237,8 @@ typedef struct BDRVQcowState {
 
     Qcow2Cache* l2_table_cache;
     Qcow2Cache* refcount_block_cache;
+    QEMUTimer *cache_clean_timer;
+    unsigned cache_clean_interval;
 
     uint8_t *cluster_cache;
     uint8_t *cluster_data;
@@ -581,6 +584,7 @@ int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     Qcow2Cache *dependency);
 void qcow2_cache_depends_on_flush(Qcow2Cache *c);
 
+void qcow2_cache_clean_unused(BlockDriverState *bs, Qcow2Cache *c);
 int qcow2_cache_empty(BlockDriverState *bs, Qcow2Cache *c);
 
 int qcow2_cache_get(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 863ffea..f42338d 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1538,6 +1538,9 @@
 # @refcount-cache-size:   #optional the maximum size of the refcount block cache
 #                         in bytes (since 2.2)
 #
+# @cache-clean-interval:  #optional clean unused entries in the L2 and refcount
+#                         caches. The interval is in seconds (since 2.4)
+#
 # Since: 1.7
 ##
 { 'struct': 'BlockdevOptionsQcow2',
@@ -1549,7 +1552,8 @@
             '*overlap-check': 'Qcow2OverlapChecks',
             '*cache-size': 'int',
             '*l2-cache-size': 'int',
-            '*refcount-cache-size': 'int' } }
+            '*refcount-cache-size': 'int',
+            '*cache-clean-interval': 'int' } }
 
 
 ##
-- 
2.1.4

Re: [Qemu-devel] [PATCH 2/3] qcow2: add option to clean unused cache entries after some time

[Kevin Wolf]

Am 29.05.2015 um 11:24 hat Alberto Garcia geschrieben:
> This adds a new 'cache-clean-interval' option that cleans all qcow2
> cache entries that haven't been used in a certain interval, given in
> seconds.
> 
> This allows setting a large L2 cache size so it can handle scenarios
> with lots of I/O and at the same time use little memory during periods
> of inactivity.
> 
> This feature currently relies on MADV_DONTNEED to free that memory, so
> it is not useful in systems that don't follow that behavior.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>

> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 863ffea..f42338d 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1538,6 +1538,9 @@
>  # @refcount-cache-size:   #optional the maximum size of the refcount block cache
>  #                         in bytes (since 2.2)
>  #
> +# @cache-clean-interval:  #optional clean unused entries in the L2 and refcount
> +#                         caches. The interval is in seconds (since 2.4)

Should add that 0 means disabling the cleaning, and 0 is the default.

Kevin

[Qemu-devel] [PATCH 3/3] qcow2: reorder fields in Qcow2CachedTable to reduce padding

[Alberto Garcia]

Changing the current ordering saves 8 bytes per cache entry in x86_64.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/qcow2-cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index a215f5b..43590ff 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -31,9 +31,9 @@
 
 typedef struct Qcow2CachedTable {
     int64_t  offset;
-    bool     dirty;
     uint64_t lru_counter;
     int      ref;
+    bool     dirty;
 } Qcow2CachedTable;
 
 struct Qcow2Cache {
-- 
2.1.4

Re: [Qemu-devel] [PATCH 3/3] qcow2: reorder fields in Qcow2CachedTable to reduce padding

[Kevin Wolf]

Am 29.05.2015 um 11:24 hat Alberto Garcia geschrieben:
> Changing the current ordering saves 8 bytes per cache entry in x86_64.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>

Reviewed-by: Kevin Wolf <kwolf@redhat.com>