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Catalog Cache Hitrate has fallen below 70 percent

former_member246694
Participant
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Dear All,

In my SCM development system Catalog Cache Hitrate has fallen below 70 percent.

Kindly guide me to make it reach beyond 90 %.

Current parameters relevant are

Cat_cache_supply= 10000

Maxusertasks = 74

Cachememorysize=1024000

let me know if you need any further info.

Regards,

kiran

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Answers (1)

Answers (1)

lbreddemann
Active Contributor
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> In my SCM development system Catalog Cache Hitrate has fallen below 70 percent.

> Kindly guide me to make it reach beyond 90 %.

>

> Current parameters relevant are

>

> Cat_cache_supply= 10000

> Maxusertasks = 74

> Cachememorysize=1024000

It's your dev system.

The catalog cache hitratio recommendation is just that: a recommendation.

It is based on observation we (SAP) made on well running productive systems.

So, there's just no need to worry about that.

Besides, neither liveCache nor MaxDB provide any reasonable way to determine:

a) what exactly causes the cache misses (and thereby lowers the hit ratio)

and

b) to measure the impact this has to the total performance of the system.

If this would be my development system, I'd care way more about e.g. backup/recovery (try, try, try!) and forget about such nonsense counters...

just my 2 pence

regards,

Lars

former_member246694
Participant
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Dear Lars,

I understand DEV system is not critical.

But Im just ambitious to know prior to the situation that might occur in a Prod system.

Regards,

Kiran

lbreddemann
Active Contributor
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> understand DEV system is not critical.

I didn't say or mean that.

I just wrote that the workload, with was assumed when the recommendation for the hit ratio was given, is not present on a dev system.

> But Im just ambitious to know prior to the situation that might occur in a Prod system.

Well, the standard recommendation for that situation is to increase the catalog cache.

However, this will be a blindfolded action, since you cannot know what ws the reason for the low hit ratio.

Usually only liveCache development is able to answer this.

regards,

Lars

former_member524429
Active Contributor
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Hi,

Catalog Cache is dynamic in nature and the space within catalog cache is allocated and released on demand as per the User session. The low hit rate of cache depends on the application usage and load.

As per [SAP Note 719652 - Setting initial parameters for liveCache 7.5, 7.6, 7.7|https://service.sap.com/sap/support/notes/719652] and SAP Note [433115|https://service.sap.com/sap/support/notes/433115] the recommended value of CAT_CACHE_SUPPLY parameter is 40*MAXUSERTASKS. In your case it is more than recommendation. Also look at the explanation given by Lars Breddemann in this useful thread [catalog cache hit rate, how to increase?|].

Also consider the below mentioned calculation: (Reference : [MaxDB Kernel Parameters release 7.6|http://maxdb.sap.com/training/internals_7.6/kernel_parameters_EN_76.pdf]--> Catalog Cache)

If (Maxusertasks + 1) / CATCACHE_MINSIZE * 8192 > CAT_CACHE_SUPPLY

CAT_CACHE_SUPPLY = (Maxusertasks + 1) / CATCACHE_MINSIZE * 8192

CATCACHE_MINSIZE * 8192 / MAXUSERTASKS + 1 <= CAT_CACHE_SUPPLY

Also, As per [SAP Note 819641 - FAQ: SAP MaxDB Performance|https://service.sap.com/sap/support/notes/819641]

Catalog Cache should be 85% in a balanced system. 
It is not a cause for concern if the catalog hit rate is temporarily lower, 
since accesses to the system tables and an active command monitor can temporarily impair the hit rate. 
If the catalog cache is busy, the pages are paged out in the data cache rather than on the hard disk.

Regards,

Bhavik G. Shroff

former_member246694
Participant
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Thank you Lara and Bhavik for the information provided.