Hello Gluster community,
My name is Barak and I’ve joined RH gluster development in August.
Shortly after my arrival, I’ve identified a potential problem with gluster’s logging mechanism and I’d like to bring the matter up for discussion.
The general concept of the current mechanism is that every worker thread that needs to log a message has to contend for a mutex which guards the log file, write the message and, flush the data and then release the mutex.
I see two design / implementation problems with that mechanism:
The mutex that guards the log file is likely under constant contention.
The fact that each worker thread perform the IO by himself, thus slowing his "real" work.
Initial tests, done by removing logging from the regression testing, shows an improvement of about 20% in run time. This indicates we’re taking a pretty heavy performance hit just because of the logging activity.
In addition to these problems, the logging module is due for an upgrade:
There are dozens of APIs in the logger, much of them are deprecated - this makes it very hard for new developers to keep evolving the project.
One of the key points for Gluster-X, presented in October at Bangalore, is the switch to a structured logging all across gluster.
Given these points, I believe we’re in a position that allows us to upgrade the logging mechanism by both switching to structured logging across the project AND replacing the logging system itself, thus “killing two birds with one stone”.
Moreover, if the upgrade is successful, the new logger mechanism might be adopted by other teams in Red Hat, which lead to uniform logging activity across different products.
I’d like to propose a logging utility I’ve been working on for the past few weeks.
This project is still a work in progress (and still much work needs to be done in it), but I’d like to bring this matter up now so if the community will want to advance on that front, we could collaborate and shape the logger to best suit the community’s needs.
An overview of the system:
The logger provides several (number and size are user-defined) pre-allocated buffers which threads can 'register' to and receive a private buffer. In addition, a single, shared buffer is also pre-allocated (size is user-defined). The number of buffers and their size is modifiable at runtime (not yet implemented).
Worker threads write messages in one of 3 ways that will be described next, and an internal logger threads constantly iterates the existing buffers and drains the data to the log file.
As all allocations are allocated at the initialization stage, no special treatment it needed for "out of memory" cases.
The following writing levels exist:
Level 1 - Lockless writing: Lockless writing is achieved by assigning each thread a private ring buffer. A worker threads write to that buffer and the logger thread drains that buffer into a log file.
In case the private ring buffer is full and not yet drained, or in case the worker thread has not registered for a private buffer, we fall down to the following writing methods:
Level 2 - Shared buffer writing: The worker thread will write it's data into a buffer that's shared across all threads. This is done in a synchronized manner.
In case the private ring buffer is full and not yet drained AND the shared ring buffer is full and not yet drained, or in case the worker thread has not registered for a private buffer, we fall down to the last writing method:
Level 3 - Direct write: This is the slowest form of writing - the worker thread directly write to the log file.
The idea behind this utility is to reduce as much as possible the impact of logging on runtime. Part of this reduction comes at the cost of having to parse and reorganize the messages in the log files using a dedicated tool (yet to be implemented) as there is no guarantee on the order of logged messages.
The full logger project is hosted on: https://github.com/BarakSason/Lockless_Logger
For project documentation visit: https://baraksason.github.io/Lockless_Logger/
I thank you all for reading through my suggestion and I’m looking forward to your feedback,
Gluster Storage Development
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