Scaling Stash

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This page discusses performance and hardware considerations when using Stash Server.

Note that Stash Data Center, not discussed on this page, uses a cluster of Stash nodes to provide Active/Active failover, and is the deployment option of choice for larger enterprises that require high availability and performance at scale.

Hardware requirements

The type of hardware you require to run Stash depends on a number of factors:

  • The number and frequency of clone operations. Cloning a repository is one of the most demanding operations. One major source of clone operations is continuous integration. When your CI builds involve multiple parallel stages, Stash will be asked to perform multiple clones concurrently, putting significant load on your system.
  • The size of your repositories – there are many operations in Stash that require more memory and more CPUs when working with very large repositories. Furthermore, huge Git repositories (larger than a few GBs) are likely to impact the performance of the Git client – see this discussion.
  • The number of users.

The following are rough guidelines for choosing your hardware:

  • Estimate the number of concurrent clones that are expected to happen regularly (look at continuous integration). Add one CPU for every 2 concurrent clone operations. Note that enabling the SCM Cache Plugin (bundled with Stash from version 2.5.0) can help to reduce the cloning load on the Stash server due to CI polling. See Scaling Stash for Continuous Integration performance.
  • Estimate or calculate the average repository size and allocate 1.5 x number of concurrent clone operations x min(repository size, 700MB) of memory.

Understanding Stash's resource usage

Most of the things you do in Stash involve both the Stash server and one or more Git processes created by Stash. For instance, when you view a file in the Stash web application, Stash processes the incoming request, performs permission checks, creates a Git process to retrieve the file contents and formats the resulting webpage. In serving most pages, both the Stash server and Git processes are involved. The same is true for the 'hosting' operations: pushing your commits to Stash, cloning a repository from Stash or fetching the latest changes from Stash. 

As a result, when configuring Stash for performance, CPU and memory consumption for both Stash and Git should be taken into account.

Stash basic flow

Memory

When deciding on how much memory to allocate for Stash, the most important factor to consider is the amount of memory required for Git. Some Git operations are fairly expensive in terms of memory consumption, most notably the initial push of a large repository to Stash and cloning large repositories from Stash. For large repositories, it is not uncommon for Git to use up to 500 MB of memory during the clone process. The numbers vary from repository to repository, but as a rule of thumb 1.5 x the repository size on disk (contents of the .git/objects directory) is a rough estimate of the required memory for a single clone operation for repositories up to 400 MB. For larger repositories, memory usage flattens out at about 700 MB.

The clone operation is the most memory intensive Git operation. Most other Git operations, such as viewing file history, file contents and commit lists are lightweight by comparison.

Stash has been designed to have fairly constant memory usage. Any pages that could show large amounts of data (e.g. viewing the source of a multi-megabyte file) perform incremental loading or have hard limits in place to prevent Stash from holding on to large amounts of memory at any time. In general, the default memory settings (max. 768 MB) should be sufficient to run Stash. The maximum amount of memory available to Stash can be configured in setenv.sh or setenv.bat.

The memory consumption of Git is not managed by the memory settings in setenv.sh or setenv.bat. The Git processes are executed outside of the Java virtual machine, and as a result the JVM memory settings do not apply to Git.

CPU

In Stash, much of the heavy lifting is delegated to Git. As a result, when deciding on the required hardware to run Stash, the CPU usage of the Git processes is the most important factor to consider. And, as is the case for memory usage, cloning large repositories is the most CPU intensive Git operation. When you clone a repository, Git on the server side will create a pack file (a compressed file containing all the commits and file versions in the repository) that is sent to the client. While preparing a pack file, CPU usage will go up to 100% for one CPU.

Encryption (either SSH or HTTPS) will have a significant CPU overhead if enabled.  As for which of SSH or HTTPS is to be preferred, there's no clear winner, each has advantages and disadvantages as described in the following table.

  HTTP HTTPS SSH
Encryption

No CPU overhead for encryption, but plaintext transfer and basic auth may be unacceptable for security.

Encryption has CPU overhead, but this can be offloaded to a separate proxy server (if the secure connection is terminated there).

Encryption has CPU overhead.

Authentication

Authentication is slower – it requires remote authentication with the LDAP or Crowd server.

Authentication is much faster – it only requires a simple lookup.

Cloning

Cloning a repository is slightly slower – it takes at least 2 and sometimes more requests, each of which needs authentication and permission checks. The extra overhead is small though - usually in the 10-100ms range

Cloning a repository takes only a single request.

Clones examined

Since cloning a repository is the most demanding operation in terms of CPU and memory, it is worthwhile analyzing the clone operation a bit closer. The following graphs show the CPU and memory usage of a clone of a 220 MB repository:

Git process (blue line)

  • CPU usage goes up to 100% while the pack file is created on the server side.
  • CPU peaks at 120% when the pack file is compressed (multiple CPUs used).
  • CPU drops back to 0.5% while the pack file is sent back to the client.

Stash (red line)

  • CPU usage briefly peaks at 30% while the clone request is processed.
  • CPU drops back to 0% while Git prepares the pack file.
  • CPU hovers around 1% while the pack file is sent to the client.

Git process (blue line)

  • Memory usage slowly climbs to 270 MB while preparing the pack file.
  • Memory stays at 270 MB while the pack file is transmitted to the client.
  • Memory drops back to 0 when the pack file transmission is complete.

Stash (red line)

  • Memory usage hovers around 800 MB and is not affected by the clone operation.

This graph shows how concurrency affects average response times for clones:

  • Vertical axis: average response times.
  • Horizontal axis: number of concurrent clone operations.
The measurements for this graph were done on a 4 CPU server with 12 GB of memory.
Response times become exponentially worse as the number of concurrent clone operations
exceed the number of CPUs.

Configuring Stash scaling options and system properties

Stash limits the number of Git operations that can be executed concurrently, to prevent the performance for all clients dropping below acceptable levels. These limits can be adjusted – see Stash config properties

Database requirements

The size of the database required for Stash depends in large part on the number of repositories and the number of commits in those repositories.

A very rough guideline is: 100 + ((total number of commits across all repos) / 2500) MB.

So, for example, for 20 repositories with an average of 25,000 commits each, the database would need 100 + (20 * 25,000 / 2500) = 300MB.

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