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+<!--
+title: "One Way Allocator"
+custom_edit_url: "https://github.com/netdata/netdata/edit/master/libnetdata/onewayalloc/README.md"
+sidebar_label: "One way allocator"
+learn_status: "Published"
+learn_topic_type: "Tasks"
+learn_rel_path: "Developers/libnetdata"
+-->
+
+# One Way Allocator
+
+This is a very fast single-threaded-only memory allocator, that minimized system calls
+when a lot of memory allocations needs to be made to perform a task, which all of them
+can be freed together when the task finishes.
+
+It has been designed to be used for netdata context queries.
+
+For netdata to perform a context query, it builds a virtual chart, a chart that contains
+all the dimensions of the charts having the same context. This process requires allocating
+several structures for each of the dimensions to attach them to the virtual chart. All
+these data can be freed immediately after the query finishes.
+
+## How it works
+
+1. The caller calls `ONEWAYALLOC *owa = onewayalloc_create(sizehint)` to create an OWA.
+   Internally this allocates the first memory buffer with size >= `sizehint`.
+   If `sizehint` is zero, it will allocate 1 hardware page (usually 4kb).
+   No need to check for success or failure. As with `mallocz()` in netdata, a `fatal()`
+   will be called if the allocation fails - although this will never fail, since Linux
+   does not really check if there is memory available for `mmap()` calls.
+   
+2. The caller can then perform any number of the following calls to acquire memory:
+   - `onewayalloc_mallocz(owa, size)`, similar to `mallocz()`
+   - `onewayalloc_callocz(owa, nmemb, size)`, similar to `callocz()`
+   - `onewayalloc_strdupz(owa, string)`, similar to `strdupz()`
+   - `onewayalloc_memdupz(owa, ptr, size)`, similar to `mallocz()` and then `memcpy()`
+   
+3. Once the caller has done all the work with the allocated buffers, all memory allocated 
+   can be freed with `onewayalloc_destroy(owa)`.
+
+## How faster it is?
+
+On modern hardware, for any single query the performance improvement is marginal and not
+noticeable at all.
+
+We performed the following tests using the same huge context query (1000 charts,
+100 dimensions each = 100k dimensions)
+
+1. using `mallocz()`, 1 caller, 256 queries (sequential)
+2. using `mallocz()`, 256 callers, 1 query each (parallel)
+3. using `OWA`, 1 caller, 256 queries (sequential)
+4. using `OWA`, 256 callers, 1 query each (parallel)
+
+Netdata was configured to use 24 web threads on the 24 core server we used.
+
+The results are as follows:
+
+### sequential test
+
+branch|transactions|time to complete|transaction rate|average response time|min response time|max response time
+:---:|:---:|:---:|:---:|:---:|:---:|:---:|
+`malloc()`|256|322.35s|0.79/sec|1.26s|1.01s|1.87s
+`OWA`|256|310.19s|0.83/sec|1.21s|1.04s|1.63s
+
+For a single query, the improvement is just marginal and not noticeable at all.
+
+### parallel test
+
+branch|transactions|time to complete|transaction rate|average response time|min response time|max response time
+:---:|:---:|:---:|:---:|:---:|:---:|:---:|
+`malloc()`|256|84.72s|3.02/sec|68.43s|50.20s|84.71s
+`OWA`|256|39.35s|6.51/sec|34.48s|20.55s|39.34s
+
+For parallel workload, like the one executed by netdata.cloud, `OWA` provides a 54% overall speed improvement (more than double the overall
+user-experienced speed, including the data query itself).