[Qemu-devel] [PATCH v3 0/2] Give the refcount cache the minimum possible size by default

[Qemu-devel] [PATCH v3 0/2] Give the refcount cache the minimum possible size by default

[Alberto Garcia]

Hi,

we talked about this the other day, so here are the patches to change
the default cache sizes in qcow2.

Without this patch:

 * refcount-cache-size = l2-cache-size / 4

unless otherwise specified by the user. This is wasteful, the refcount
cache is accessed sequentially during normal I/O, so there's no point
in caching more tables. I measured the effect on the refcount cache
size when populating an empty qcow2 image using random writes, and
there's no difference between having the minimum or the maximum
sizes(*).

With this patch:

 * refcount-cache-size is always 4 clusters by default (the minimum)

 * If "cache-size" is set then l2-cache-size is set to the maximum if
   possible (disk_size * 8 / cluster_size) and the remainder is
   assigned to the refcount cache.

Regards,

Berto

(*) there is, actually: having a very large cache can even make the
    I/O slightly slower, because the larger the cache the longer it
    takes longer to find a cached entry. I only noticed this under
    tmpfs anyway.

Changes:
v3:
- Mention that if you use internal snapshots you may want to increase
  the cache size [Max]

v2: https://lists.gnu.org/archive/html/qemu-block/2018-03/msg00822.html
- s/overriden/overridden/ (in both patches)

v1: https://lists.gnu.org/archive/html/qemu-block/2018-03/msg00709.html
- Initial release

Alberto Garcia (2):
  qcow2: Give the refcount cache the minimum possible size by default
  docs: Document the new default sizes of the qcow2 caches

 block/qcow2.c              | 31 +++++++++++++++++++------------
 block/qcow2.h              |  4 ----
 docs/qcow2-cache.txt       | 33 ++++++++++++++++-----------------
 tests/qemu-iotests/137.out |  2 +-
 4 files changed, 36 insertions(+), 34 deletions(-)

-- 
2.11.0

[Qemu-devel] [PATCH v3 1/2] qcow2: Give the refcount cache the minimum possible size by default

[Alberto Garcia]

The L2 and refcount caches have default sizes that can be overridden
using the l2-cache-size and refcount-cache-size (an additional
parameter named cache-size sets the combined size of both caches).

Unless forced by one of the aforementioned parameters, QEMU will set
the unspecified sizes so that the L2 cache is 4 times larger than the
refcount cache.

This is based on the premise that the refcount metadata needs to be
only a fourth of the L2 metadata to cover the same amount of disk
space. This is incorrect for two reasons:

 a) The amount of disk covered by an L2 table depends solely on the
    cluster size, but in the case of a refcount block it depends on
    the cluster size *and* the width of each refcount entry.
    The 4/1 ratio is only valid with 16-bit entries (the default).

 b) When we talk about disk space and L2 tables we are talking about
    guest space (L2 tables map guest clusters to host clusters),
    whereas refcount blocks are used for host clusters (including
    L1/L2 tables and the refcount blocks themselves). On a fully
    populated (and uncompressed) qcow2 file, image size > virtual size
    so there are more refcount entries than L2 entries.

Problem (a) could be fixed by adjusting the algorithm to take into
account the refcount entry width. Problem (b) could be fixed by
increasing a bit the refcount cache size to account for the clusters
used for qcow2 metadata.

However this patch takes a completely different approach and instead
of keeping a ratio between both cache sizes it assigns as much as
possible to the L2 cache and the remainder to the refcount cache.

The reason is that L2 tables are used for every single I/O request
from the guest and the effect of increasing the cache is significant
and clearly measurable. Refcount blocks are however only used for
cluster allocation and internal snapshots and in practice are accessed
sequentially in most cases, so the effect of increasing the cache is
negligible (even when doing random writes from the guest).

So, make the refcount cache as small as possible unless the user
explicitly asks for a larger one.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2.c              | 31 +++++++++++++++++++------------
 block/qcow2.h              |  4 ----
 tests/qemu-iotests/137.out |  2 +-
 3 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index ef68772aca..091088e09e 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -802,23 +802,30 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
         } else if (refcount_cache_size_set) {
             *l2_cache_size = combined_cache_size - *refcount_cache_size;
         } else {
-            *refcount_cache_size = combined_cache_size
-                                 / (DEFAULT_L2_REFCOUNT_SIZE_RATIO + 1);
-            *l2_cache_size = combined_cache_size - *refcount_cache_size;
+            uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
+            uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
+            uint64_t min_refcount_cache =
+                (uint64_t) MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
+
+            /* Assign as much memory as possible to the L2 cache, and
+             * use the remainder for the refcount cache */
+            if (combined_cache_size >= max_l2_cache + min_refcount_cache) {
+                *l2_cache_size = max_l2_cache;
+                *refcount_cache_size = combined_cache_size - *l2_cache_size;
+            } else {
+                *refcount_cache_size =
+                    MIN(combined_cache_size, min_refcount_cache);
+                *l2_cache_size = combined_cache_size - *refcount_cache_size;
+            }
         }
     } else {
-        if (!l2_cache_size_set && !refcount_cache_size_set) {
+        if (!l2_cache_size_set) {
             *l2_cache_size = MAX(DEFAULT_L2_CACHE_BYTE_SIZE,
                                  (uint64_t)DEFAULT_L2_CACHE_CLUSTERS
                                  * s->cluster_size);
-            *refcount_cache_size = *l2_cache_size
-                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
-        } else if (!l2_cache_size_set) {
-            *l2_cache_size = *refcount_cache_size
-                           * DEFAULT_L2_REFCOUNT_SIZE_RATIO;
-        } else if (!refcount_cache_size_set) {
-            *refcount_cache_size = *l2_cache_size
-                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
+        }
+        if (!refcount_cache_size_set) {
+            *refcount_cache_size = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
         }
     }
 
diff --git a/block/qcow2.h b/block/qcow2.h
index adf5c3950f..01b5250415 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -77,10 +77,6 @@
 #define DEFAULT_L2_CACHE_CLUSTERS 8 /* clusters */
 #define DEFAULT_L2_CACHE_BYTE_SIZE 1048576 /* bytes */
 
-/* The refblock cache needs only a fourth of the L2 cache size to cover as many
- * clusters */
-#define DEFAULT_L2_REFCOUNT_SIZE_RATIO 4
-
 #define DEFAULT_CLUSTER_SIZE 65536
 
 
diff --git a/tests/qemu-iotests/137.out b/tests/qemu-iotests/137.out
index e28e1eadba..96724a6c33 100644
--- a/tests/qemu-iotests/137.out
+++ b/tests/qemu-iotests/137.out
@@ -22,7 +22,7 @@ refcount-cache-size may not exceed cache-size
 L2 cache size too big
 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
-L2 cache size too big
+Refcount cache size too big
 Conflicting values for qcow2 options 'overlap-check' ('constant') and 'overlap-check.template' ('all')
 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all
 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all
-- 
2.11.0

[Qemu-devel] [PATCH v3 2/2] docs: Document the new default sizes of the qcow2 caches

[Alberto Garcia]

We have just reduced the refcount cache size to the minimum unless
the user explicitly requests a larger one, so we have to update the
documentation to reflect this change.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
 docs/qcow2-cache.txt | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 170191a242..8a09a5cc5f 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -116,31 +116,30 @@ There are three options available, and all of them take bytes:
 "refcount-cache-size":   maximum size of the refcount block cache
 "cache-size":            maximum size of both caches combined
 
-There are two things that need to be taken into account:
+There are a few things that need to be taken into account:
 
  - Both caches must have a size that is a multiple of the cluster size
    (or the cache entry size: see "Using smaller cache sizes" below).
 
- - If you only set one of the options above, QEMU will automatically
-   adjust the others so that the L2 cache is 4 times bigger than the
-   refcount cache.
+ - The default L2 cache size is 8 clusters or 1MB (whichever is more),
+   and the minimum is 2 clusters (or 2 cache entries, see below).
 
-This means that these options are equivalent:
+ - The default (and minimum) refcount cache size is 4 clusters.
 
-   -drive file=hd.qcow2,l2-cache-size=2097152
-   -drive file=hd.qcow2,refcount-cache-size=524288
-   -drive file=hd.qcow2,cache-size=2621440
+ - If only "cache-size" is specified then QEMU will assign as much
+   memory as possible to the L2 cache before increasing the refcount
+   cache size.
 
-The reason for this 1/4 ratio is to ensure that both caches cover the
-same amount of disk space. Note however that this is only valid with
-the default value of refcount_bits (16). If you are using a different
-value you might want to calculate both cache sizes yourself since QEMU
-will always use the same 1/4 ratio.
+Unlike L2 tables, refcount blocks are not used during normal I/O but
+only during allocations and internal snapshots. In most cases they are
+accessed sequentially (even during random guest I/O) so increasing the
+refcount cache size won't have any measurable effect in performance
+(this can change if you are using internal snapshots, so you may want
+to think about increasing the cache size if you use them heavily).
 
-It's also worth mentioning that there's no strict need for both caches
-to cover the same amount of disk space. The refcount cache is used
-much less often than the L2 cache, so it's perfectly reasonable to
-keep it small.
+Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
+L2 cache size. This resulted in unnecessarily large caches, so now the
+refcount cache is as small as possible unless overridden by the user.
 
 
 Using smaller cache entries
-- 
2.11.0

Re: [Qemu-devel] [PATCH v3 2/2] docs: Document the new default sizes of the qcow2 caches

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 589 bytes --]

On 04/17/2018 07:37 AM, Alberto Garcia wrote:
> We have just reduced the refcount cache size to the minimum unless
> the user explicitly requests a larger one, so we have to update the
> documentation to reflect this change.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  docs/qcow2-cache.txt | 33 ++++++++++++++++-----------------
>  1 file changed, 16 insertions(+), 17 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 619 bytes --]

Re: [Qemu-devel] [PATCH v3 0/2] Give the refcount cache the minimum possible size by default

[Max Reitz]

[-- Attachment #1: Type: text/plain, Size: 2056 bytes --]

On 2018-04-17 14:37, Alberto Garcia wrote:
> Hi,
> 
> we talked about this the other day, so here are the patches to change
> the default cache sizes in qcow2.
> 
> Without this patch:
> 
>  * refcount-cache-size = l2-cache-size / 4
> 
> unless otherwise specified by the user. This is wasteful, the refcount
> cache is accessed sequentially during normal I/O, so there's no point
> in caching more tables. I measured the effect on the refcount cache
> size when populating an empty qcow2 image using random writes, and
> there's no difference between having the minimum or the maximum
> sizes(*).
> 
> With this patch:
> 
>  * refcount-cache-size is always 4 clusters by default (the minimum)
> 
>  * If "cache-size" is set then l2-cache-size is set to the maximum if
>    possible (disk_size * 8 / cluster_size) and the remainder is
>    assigned to the refcount cache.
> 
> Regards,
> 
> Berto
> 
> (*) there is, actually: having a very large cache can even make the
>     I/O slightly slower, because the larger the cache the longer it
>     takes longer to find a cached entry. I only noticed this under
>     tmpfs anyway.
> 
> Changes:
> v3:
> - Mention that if you use internal snapshots you may want to increase
>   the cache size [Max]
> 
> v2: https://lists.gnu.org/archive/html/qemu-block/2018-03/msg00822.html
> - s/overriden/overridden/ (in both patches)
> 
> v1: https://lists.gnu.org/archive/html/qemu-block/2018-03/msg00709.html
> - Initial release
> 
> Alberto Garcia (2):
>   qcow2: Give the refcount cache the minimum possible size by default
>   docs: Document the new default sizes of the qcow2 caches
> 
>  block/qcow2.c              | 31 +++++++++++++++++++------------
>  block/qcow2.h              |  4 ----
>  docs/qcow2-cache.txt       | 33 ++++++++++++++++-----------------
>  tests/qemu-iotests/137.out |  2 +-
>  4 files changed, 36 insertions(+), 34 deletions(-)

Thanks, applied to my block-next branch:

https://github.com/XanClic/qemu/commits/block-next

Max


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]