April 18, 2011
Comparing performance is a game of how to do apples-to-apples comparison and yet make your system look better. The simplest trick is to chose a hardware or a software that favors your system. And use the same platform for the competing system. Then under identical conditions, using same benchmark, your system would obviously perform better. If you look at the experiment as a whole, it’s a clean apples-to-apples comparison. Many competitive papers published or sponsored by vendors may be such studies.
To understand the real game, one needs to know more about how the two particular softwares are designed. How they work with different underlying hardware. Each system has unique features which are designed to perform better under specific applications. For example, one may be using on-board cache to give better performance while other may be using multiple paths to the storage to give better performance. Any differentiating features between the two softwares can be exploited to tilt the level of the playing field. By ‘correctly’ choosing the underlying hardware one of them can be shown in better light than the other.
That is why even though system performance is a very rigorous subject, it’s application to compare performance of commercial systems is a subtle art.
So, the next time you come across a ‘performance comparison’, be sure to look at the conditions under which the performance measurement was carried out.
April 18, 2011
System performance is a general term. It means, how the system (hardware, software or both) is doing under some specified conditions. Or how best can system performs under real load conditions. An appropriate measure for the performance should be chosen based on the most valuable aspect given any situation. There isn’t a single common parameter that fits all situations.
One of the most commonly used parameters for measuring performance is the number of transactions the system performs in one second or, ‘transactions/sec’. This simply counts the number of operations a system can do in one second. This parameter is all right if it is required that the system deliver maximum number of transactions.
Another frequently used parameter is – how long does a specified operation take to complete. This is ‘inverse’ of the previous parameter. Here the interest is in the average time (say in milliseconds) taken to complete a single transaction. This may be of interest if one wants a measure of how FAST the system serves users.
There are other performance parameters as well. For example, how efficient the code is (especially, code which does computation), then one is interested in looking at the number of cpu cycles spent in doing transactions every second or, ‘transactions/sec/cpu’. An efficient code will deliver greater transactions per second while using less cpu.
If the interest is in the measure of raw data performance – written or read from the disk then the appropriate parameter is the number of blocks written/read form the disk. If one is studying the performance of data-base then one may want to look at how many SQL operations (or scripts) are executed per second.
For a web-server, it could be the time taken to load the pages from multiple users.
The measure of performance should capture the quality that is being sought in the system. The performance numbers or claims may prove misleading simply because one is not using the correct measure of performance. It is thus very important to choose the correct measure of system performance.
April 12, 2011
In any system under performance consideration, there are 3 important timescales.
- The timescale at which the CPU works
- The timescale at which the Memory can be accessed
- The timescale at which data can be stored on disk
These three have widely different timescales in a traditional computer setup. CPU runs at nano second resolution, memory at micro-second and Disks at milli-second.
This makes it difficult to estimate the true efficiency of code. Because, the rate at which the instructions can be processed also now depends on the rate at which the memory is accessed and the disk is utilized. Hence, to truly measure the code efficiency one needs to make sure that the disk response times do not play a major role in the flow of instructions. In other words on e should take care of removing the IO bottlenecks before analyzing the performance of code.
In presence of strong influence of disk IOs the measurements will be biased by the characteristics of the storage system. The order of magnitude difference between the response times of disks and memory and memory and CPU makes it even more difficult to remove the IO bottleneck. While the CPUs have become faster in the last few years and there are now CPUs with multiple cores the memory and disk access speeds haven’t kept pace. As a result the contrast between the response times of CPU, Memory and Storage has widened.
April 9, 2011
Cloud computing is now synonymous with Flexible Provisioning and Scale. Find out below if you are taking full advantage of cloud computing.
The As Is deployment – lowest adoption cost, reasonable benefits:
Move the server application “as is” to a cloud server. This is nothing but a co-located server, at Amazon for example. The provisioning and maintenance of the application is still a self driven task.
The win is in the dynamic on demand provisioning. Easy to compute the ROI here. Let us say that your application needs to be available all year round – but cater to seasonal demands. Say it costs $400 to host your application to cater to peak demand. You would end up paying 12*400 = $4800 per annum to keep your application up. Most of the time it would be under utilized. Cloud computing has made it really simple to change your compute capacity as easily as setting a reminder in your out look calender. With amazon or google, you could just log into the admin panel and say that you need additional resources only on certain dates. At the end of the month you get billed for the amount of resources you actually consume.
The Managed RDBMS deployment – reasonably low adoption cost, reasonable benefits:
A lot of work has to be done to ensure that the application is available. i.e. a replication strategy and policy to keep the database available. This is still a lot of effort and money. The alternative is a managed RDBMS, where the provider (amazon or google) manages the database. They worry about keeping the data safe from being lost. Much harder to do the ROI here – as the time spent in managing this would have to be offset against opportunity costs. Note that there would be some amount of code restructuring (not a lot) to get this going. An example of this is the Amazon MySQL RDS. At the time of writing, google is yet to announce the availability of their hosted sql service.
The Application Rewrite – highest adoption cost, highest benefits (arguably)
If your goal is to write an application which scales very well then you should consider a complete application rewrite to take advantage of the storage APIs. Hosted RDBMS is still a single machine (or a cluster) running a database server – with bottlenecks – be it memory, cpu, networ or disk.
Cloud computing offers storage APIs to access and manage data unlike traditional methods of file or rdbms storage. Because of the underlying architectural differences, cloud datastore offers better scalability – http://labs.google.com/papers/bigtable.html.