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Application Level Caching

August 26, 2011 Leave a comment

Everyone here probably knows the various levels of caching that exist on a modern computer: From multiple CPU caches through to disk cache and even caching in the database engine itself. If you want to quickly touch up on some caching concepts/terminology, check out this short slide deck from Serhiy Oplakanets on Caching Basics

What I’m going to do shortly is outline some other methods of gaining significant performance improvements on your UniData and UniVerse systems.

There really isn’t anything special outside of U2 that you will need to do to get benefits from this, although a few extra tricks that do require either additional hardware or OS work can give quite a boost

First, just to make sure everyone is on the same page: Since UniData and UniVerse support hash-tables as their file (table) structure, you can simply use a file as a gloried key-value store. Key-value stores are ideal for caching.

I’ve dividing this post into 4 sections:

  1. Session Level Caching
  2. Account Level Caching
  3. Improving the above (SSD and RAM Disk)
  4. In Summary

Let me know what you think.

 

Session Level Caching

 

COMMON provides a method of keeping a small in-memory cache for the entire duration of a session. Simply declare an array in a named common block and away you go.

A real world example, I’ve seen this used for when a dictionary item made a SUBR call to a subroutine that in turn would read a multitude of control items to process the original record. This dictionary item was called nightly by an external reporting tool on a large number of records.

The original solution had an unacceptable run-time and after some profiling, it was determined that the READs of the control items were the main culprit. Since it was known that the control items would not change (and should not) during the processing, it was determined that caching the control items in-memory after they were read would reduce the run-time.

The solution involved: An array of ‘x’ elements. When a control item needed to be read in, it checked this array via a simple look-up and if it existed, it used it. If not, it would read it from disk and store it in the array.

The result: 10+ hour run-time was now less than 1 hour.

 

Account Caching

 

Alright, so you have a system that needs to handle some messages (perhaps via some form of SOAP/REST web service) The majority are read requests with a few write requests for good measure.

One of these messages is to ‘Get Products’. This message returns a list of products (ID, name and latest available version) that a customer currently has.

In your system, there are 2 files used by this request. ‘CUSTOMERS’ and ‘PRODUCTS’. CUSTOMERS<30> is a multivalued list of record ids for ‘PRODUCTS’. PRODUCTS<1> is the name of the product and PRODUCTS<11> is the latest available version.

Traditionally for each ‘Get Products’ request your system would read in the appropriate record then read in all the linked records from PRODUCTS to compile the response to the query. Assuming an average customer has 10 products, the average disk reads for this query is 11

Now this query is being called a lot, all these extra disk reads and processing are beginning to cause performance impacts. Thankfully, because your database supports key-value storage, you can quickly implement a cache to sit in between the receipt of the message and the processing.

All that is needed is a new file called ‘CACHE.GETPRODUCTS’. @ID is the CUSTOMERS id requested in the query, <1> is the date requested, <2> is the time requested and <3> is the response generated

Now, when ‘Get Products’ query is received, it will first do a read of the cache file and if it exists, simply return <3>. If the entry doesn’t exist, it will hand the request/response off to the usual processing routine. The subsequent request will then be stored in the cache before being returned.

Assuming the average declared above, a cache hit will result in 1 disk read and a cache miss will result in 12 disk reads and 1 write. If – for ease of math – we treat a write equal to a read, you only need a 16.7% Cache hit rate for it to perform better. That isn’t even taking in to considering CPU usage reduction, better disk cache performance, etc.

How you handle cache invalidation is dependent on your situation. It could be as simple as clearing it every ‘x’ period, as straight forward ignoring the cache record if it is older than ‘y’ time or as complex as individually invalidating records based on when the appropriate records in CUSTOMERS or PRODUCTS change.

What has been implemented here is a cache that is available not only in the current session, but to any program running or that will be run in the account(s) that have access to this cache file.

 

Improving the above

 

Okay, so you have a more intensive system than the above and you have determined caching can help you out. The problem is, even with the caching it still doesn’t meet your requirements and disk has been determined to be the biggest bottleneck.

You have 2 next steps that can be implemented easily.

The Disk Approach

Simple drop in a shiny new SSD drive or a WD Raptor and move the cache files over there. No need to back them up, mirror them or anything else as caching files are temporary data. As long as your system is setup to recreate them if missing on start-up and treat it as a cache miss if unavailable during operation, you are all set.

The benefit here is faster disk access as well as moving the activity off on to another device/bus.

The RAM approach

Instead of adding new hardware, perhaps you’d prefer to spare 64MB of RAM to the cause. In this case, you would simply create a RAM Drive and move the cache files there. You have now essentially created a RAM based key-value store to use as your heart desires.

For an example of what type of improvements this can have, I took the DOSAC test I previously created and ran it twice. Once with the file on traditional disk and once with the file on RAM Disk. The system stats are identical to last time I ran the test, except it was on Fedora (it comes with multiple 16MB RAM disks pre-configured).

RAM DOSAC Results

RAM DOSAC Results

That’s right: Massive improvements, as expected (excuse the display text bug).

 

In Summary

 

So, keep this in mind. U2 Databases give you some great flexibility in how you implement your software. Knowing the options available is crucial to being able to get the best results.

As the saying goes, measure twice, cut once. Work out what your performance bottlenecks are then determine the best solution. Some times it is better hardware, sometimes it is code clean up. Sometimes… it might just call for caching.

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