The QlikView Cache

[hic]

QlikView has a very efficient, patented caching algorithm that effectively eliminates the calculation time for calculations that have been made before. In other words, if you use the “back” button in the toolbar, or if you happen to make a selection that you have made before, you usually get the result immediately. No calculation is necessary.

But how does it work? What is used as lookup ID?

For each object or combination of data set and selection or data sub-set and expression QlikView calculates a digital fingerprint that identifies the context. This is used as lookup ID and stored in the cache together with the result of the calculation.

Here "calculation" means both the Logical Inference and Chart calculation - or in fact, any expression anywhere. This means that both intermediate and final results of a selection are stored.

There are some peculiarities you need to know about the cache…

• The cache is global. It is used for all users and all documents. A cache entry does not belong to one specific document or one user only. So, if a user makes a selection that another user already has made, the cache is used. And if you have the same data in two different apps, one single cache entry can be used for both documents.
• Memory is not returned, when the document is unloaded. Cache entries will usually not be purged until the RAM usage is close to or has reached the lower working set limit. QlikView will then purge some entries and re-use the memory for other cache entries. This behavior sometimes makes people believe there is a memory leak in the product. But have no fear – it should be this way. So, you do not need to restart the service to clear the cache.
• The oldest cache entries are not purged first. Instead several factors are used to calculate a priority for each cache entry; factors like RAM usage, cost to calculate it again and time since the most recent usage. Entries with a combined low priority will be purged when needed. Hence, an entry that is cheap to calculate again will easily be purged, also if it recently was used. And another value that is expensive to recalculate or just uses a small amount of RAM will be kept for a much longer time.
• The cache is not cleared when running macros which I have seen some people claim.
• You need to write your expression exactly right. If the same expression is used in several places, it should be written exactly the same way – Capitalization, same number of spaces, etc. – otherwise it will not be considered to be the same expression. If you do, there should be no big performance difference between repeating the formula, referring to a different expression using the label of the expression or using the Column() function.

The cache efficiently speeds up QlikView. Basically it is a way to trade memory against CPU-time: If you put more memory in your server, you will be able to re-use more calculations and thus use less CPU-time.

HIC

Further reading on the Qlik engine internals:

Symbol Tables and Bit-Stuffed Pointers

Colors, States and State vectors

Logical Inference and Aggregations

[Anonymous]

Very helpful post.

Interesting point with the expression writing though.

I wonder if it makes a noticeable difference. Need to test.

[#3]

Hi Henric,

The cache is global.

If section access is enabled what will happen? Will it blindly won't consider cache data or It will verify whether both users are having same level authorization to data access?

Karthik

[richard_pearce6]

Thanks, very informative and concise

[simondachstr]

Thanks for the post. In the QMS API documentation, there's a method called ClearQVSCache which can "clear" the following members:

	Member name	Value	Description
	None	0	No object specified.
	License	1	Cached QlikView Server license information.
	Settings	2	Cached QlikView Server settings.
	UserDocumentList	4	Cached QlikView Server user document file structure.
	UserDocumentMetaData	8	Cached QlikView Server user document meta data.
	CALConfiguration	16	Cached QlikView Server CAL configuration.
	All	65535	All cache objects.

Can you maybe please elaborate on those cache-members and how they are different to what you described in your blog.

[chris_johnson]

Hi,

Interesting article Henric.

Does this also apply with using variables as Expressions? If I were to use the expression $(SalesFigure) in multiple charts so that this is consistent would this be considered as the same expression, and would there be any additional overhead with having to resolve the content of the variable?

Thanks,

Chris

[rbecher]

You need to write your expression exactly right.

This seems to be very interesting. I've never heared before and I wonder why the expressions aren't compared after parsing. Do comments have an effect also? I see there some ptential improvement..

- Ralf

[IAMDV]

Many thanks HIC. Interesting post and one of my favorite topic.

I agree with Ralf regarding - You need to write your expression exactly right.

Chris - I think expressions will be cached even if we use variables with dollar sign expansion. I'd think the variables are containers of expressions instead of just holding the absolute value.

Does anyone tried or considered using SSD and using the Swap File for caching?

Thanks,

DV

[barryharmsen]

Great post Henric! For those who really want to experience how much performance caching adds, I've written a little post that shows you how to turn off caching (don't forget to turn it back on!). The difference is extreme: The power of QlikView caching » The Qlik Fix! The Qlik Fix!

Ralf Becher the "your expressions need to be exactly the same" thing was covered nicely in this blog post from last year, a very interesting discovery: Performance (and other) benefits of using expression column and label references « BI Commons

[kji]

Since the database will be different for different levels of authorization, the thumbprints will not match.

[#11]

Very informative. But some questions come up:

And if you have the same data in two different apps, one single cache entry can be used for both documents.

If a 'Total' app is distributed along with a level1 loop-and-reduced and a level2 loop-and-reduced set of apps, is it really so, that cache is shared between these apps? Example: cache from level1, value1 app will be available when a user select value1 in the Total app.

And second, if level1 is present in another app used with another set of data and loaded with different qvd's, then level1 is only present once in memory?