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Merge #17751 : doc: use recommended shebang approach in documentation code block

6094222de7820d235e6e8c66e589aa71db08c077 use preferred shebang approach for documentation (hackerrdave)

Pull request description:

  Documentation update to use recommended shebang approach mentioned in the [developer notes](https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#shebang)

ACKs for top commit:
  hebasto:
    ACK 6094222de7820d235e6e8c66e589aa71db08c077, I have reviewed the code, and it looks OK, I agree it can be merged.

Tree-SHA512: fc58632f0a6fa82c7abdddfac4897f082110d647426d2b468cba6fabf6b34a015fcad47e5b26be98e629b8b0417b8781e8d89da67189e20da228b97b17f1a532

2021-07-14 18:43:55 -05:00

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Reduce Memory

There are a few parameters that can be dialed down to reduce the memory usage of dashd. This can be useful on embedded systems or small VPSes.

In-memory caches

The size of some in-memory caches can be reduced. As caches trade off memory usage for performance, reducing these will usually have a negative effect on performance.

-dbcache=<n> - the UTXO database cache size, this defaults to 450. The unit is MiB (1024).
- The minimum value for -dbcache is 4.
- A lower -dbcache makes initial sync time much longer. After the initial sync, the effect is less pronounced for most use-cases, unless fast validation of blocks is important, such as for mining.

Memory pool

In Dash Core there is a memory pool limiter which can be configured with -maxmempool=<n>, where <n> is the size in MB (1000). The default value is 300.
- The minimum value for -maxmempool is 5.
- A lower maximum mempool size means that transactions will be evicted sooner. This will affect any uses of dashd that process unconfirmed transactions.
To completely disable mempool functionality there is the option -blocksonly. This will make the client opt out of receiving (and thus relaying) transactions completely, except as part of blocks.
- Do not use this when using the client to broadcast transactions as any transaction sent will stick out like a sore thumb, affecting privacy. When used with the wallet it should be combined with -walletbroadcast=0 and -spendzeroconfchange=0. Another mechanism for broadcasting outgoing transactions (if any) should be used.
Since 0.14.0, unused memory allocated to the mempool (default: 300MB) is shared with the UTXO cache, so when trying to reduce memory usage you should limit the mempool, with the -maxmempool command line argument.

Number of peers

-maxconnections=<n> - the maximum number of connections, this defaults to 125. Each active connection takes up some memory. Only significant if incoming connections are enabled, otherwise the number of connections will never be more than 8.

Thread configuration

For each thread a thread stack needs to be allocated. By default on Linux, threads take up 8MiB for the thread stack on a 64-bit system, and 4MiB in a 32-bit system.

-par=<n> - the number of script verification threads, defaults to the number of cores in the system minus one.
-rpcthreads=<n> - the number of threads used for processing RPC requests, defaults to 4.

Linux specific

By default, since glibc 2.10, the C library will create up to two heap arenas per core. This is known to cause excessive memory usage in some scenarios. To avoid this make a script that sets MALLOC_ARENA_MAX before starting dashd:

#!/usr/bin/env bash
export MALLOC_ARENA_MAX=1
dashd

The behavior was introduced to increase CPU locality of allocated memory and performance with concurrent allocation, so this setting could in theory reduce performance. However, in Dash Core very little parallel allocation happens, so the impact is expected to be small or absent.

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