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

[Qemu-devel] [PATCH 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.

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       | 31 ++++++++++++++-----------------
 tests/qemu-iotests/137.out |  2 +-
 4 files changed, 34 insertions(+), 34 deletions(-)

-- 
2.11.0

[Qemu-devel] [PATCH 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 overriden
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>
---
 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 7472af6931..8342b0186f 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 ccb92a9696..cdf41055ae 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

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

[Eric Blake]

On 03/13/2018 10:02 AM, Alberto Garcia wrote:
> The L2 and refcount caches have default sizes that can be overriden
> 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.
> 

> 
> 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.

I like the reasoning given here.

I'd count this as a bugfix, safe even during freeze (but it's ultimately 
the maintainer's call)

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

> +++ 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;

In the old code, refcount_cache_size and l2_cache_size are both set to 
fractions of the combined size, so both are positive (even if 
combined_cache_size is too small for the minimums required)

> +            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);

but here, if combined_cache_size is smaller than min_refcount_cache,

> +                *l2_cache_size = combined_cache_size - *refcount_cache_size;

then l2_cache_size is set to a negative value.

I think you're missing bounds validations that the combined cache size 
is large enough for the minimums required.  Or maybe a slight tweak, if 
it is okay for one of the two caches to be sized at 0 (that is, if 
combined_cache_size is too small for refcount, can it instead be given 
100% to the l2 cache and let refcount be uncached)?

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

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

[Alberto Garcia]

On Tue 13 Mar 2018 07:23:36 PM CET, Eric Blake wrote:
>> +                *refcount_cache_size =
>> +                    MIN(combined_cache_size, min_refcount_cache);
>
> but here, if combined_cache_size is smaller than min_refcount_cache,
>
>> +                *l2_cache_size = combined_cache_size - *refcount_cache_size;
>
> then l2_cache_size is set to a negative value.

No, it's set to 0.

If combined == 4k and min_refcount == 256, then

   refcount_cache_size = MIN(4k, 256k) // 4k
   l2_cache_size = 4k - 4k; // 0

Then the caller ensures that it's always set to the minimum (as it did
with the previous code).

Berto

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

[Eric Blake]

On 03/13/2018 01:48 PM, Alberto Garcia wrote:
> On Tue 13 Mar 2018 07:23:36 PM CET, Eric Blake wrote:
>>> +                *refcount_cache_size =
>>> +                    MIN(combined_cache_size, min_refcount_cache);
>>
>> but here, if combined_cache_size is smaller than min_refcount_cache,
>>
>>> +                *l2_cache_size = combined_cache_size - *refcount_cache_size;
>>
>> then l2_cache_size is set to a negative value.
> 
> No, it's set to 0.
> 
> If combined == 4k and min_refcount == 256, then
> 
>     refcount_cache_size = MIN(4k, 256k) // 4k
>     l2_cache_size = 4k - 4k; // 0

Ah. Mental breakdown on my part in trying to compute (x - MIN()).

> 
> Then the caller ensures that it's always set to the minimum (as it did
> with the previous code).

So the caller will use larger than the requested limits if the requested 
limits are too small, and we are okay with calculations resulting in 0 
here.  All right, thanks for stepping me through my error; you're good 
to go with:

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

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

[Qemu-devel] [PATCH 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 | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 170191a242..6bd96cad29 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -116,31 +116,28 @@ 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.
 
-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 overriden by the user.
 
 
 Using smaller cache entries
-- 
2.11.0

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

[Eric Blake]

On 03/13/2018 10:02 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 | 31 ++++++++++++++-----------------
>   1 file changed, 14 insertions(+), 17 deletions(-)
> 

> +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 overriden by the user.

s/overriden/overridden/

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

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