Issue:
When the setting nvs cache is disabled and `settings_nvs_save` is
called, the function reads all stored setting name entries from NVS
until either finds the desired setting name entry or reaches the last
stored setting name entry.
With the settings nvs cache enabled, `settings_nvs_save` runs through
the cached setting name entries first. If the cached entry matches with
the desired one, it immediately writes the new setting value to NVS
that corresponds to the cached setting name entry.
However, if the setting name entry is not found in the cache (which is
the case for a new entry), `settings_nvs_save` reads all stored setting
name entries from NVS again. This means that even if the number of
stored entries in the settings is less than the cache size, for each
new setting entry to be stored `settings_nvs_save` will first run
through the cache, then read all stored setting name entries from NVS
and only then will pick the next free name id for this new setting name
entry and will finally store the new setting entry.
This makes the cache ineffiсient for every new entry to be stored even
when the cache size is always able to keep all setting entries that will
be stored in NVS.
Use-case:
In the Bluetooth mesh there is a Replay Protection List which keeps
sequence numbers of all nodes it received messages from. The RPL is
stored persistently in NVS. The setting name entry is the source
address of the node and the setting value entry is the sequence number.
The common use case is when RPL is quite big (for example, 255 entries).
With the current settings nvs cache implementation, every time the node
stores a new RPL entry in settings (which is the first received message
from a particular source address), `settings_nvs_save` will always check
the cache first, then also read all stored entries in NVS and only then
will figure out that this is a new entry. With every new RPL entry to be
stored this search time increases. This behavior results in much worse
performance in comparison with when the corresponding entry was already
stored. E.g. on nRF52840, with bare minimal mesh stack configuration,
when the cache is bigger than number of stored entries or close to it,
storing of 255 RPL entries takes ~25 seconds. The time of subsequent
store of 255 RPL entires is ~2 seconds with the cache.
Solution:
This commit improves the behavior of the first write by bypassing the
reading from NVS if the following conditions are met:
1. `settings_nvs_load` was called,
2. the cache was not overflowed (bigger than the number of stored
entries).
As long as these 2 conditiones are met, it is safe to skip reading from
NVS, pick the next free name id and write the value immediately.
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>
a35bb6608d