added new section about tuning libtorrent for special needs
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Arvid Norberg
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docs/tuning.rst
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docs/tuning.rst
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=================
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libtorrent manual
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=================
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:Author: Arvid Norberg, arvid@rasterbar.com
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:Version: 0.15.0
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.. contents:: Table of contents
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:depth: 2
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:backlinks: none
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tuning libtorrent
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=================
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libtorrent expose most constants used in the bittorrent engine for
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customization through the ``session_settings``. This makes it possible to
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test and tweak the parameters for certain algorithms to make a client
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that fits a wide range of needs. From low memory embedded devices to
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servers seeding thousands of torrents. The default settings in libtorrent
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are tuned for an end-user bittorrent client running on a normal desktop
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computer.
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This document describes techniques to benchmark libtorrent performance
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and how parameters are likely to affect it.
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reducing memory footprint
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=========================
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These are things you can do to reduce the memory footprint of libtorrent. You get
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some of this by basing your default ``session_settings`` on the ``min_memory_usage()``
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setting preset function.
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Keep in mind that lowering memory usage will affect performance, always profile
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and benchmark your settings to determine if it's worth the trade-off.
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The typical buffer usage of libtorrent, for a single download, with the cache
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size set to 256 blocks (256 * 16 kiB = 4 MiB) is::
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read cache: 149.5 (2392 kiB)
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write cache: 89.5 (1432 kiB)
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receive buffers: 6.5 (104 kiB)
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send buffers: 4 (64 kiB)
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hash temp: 0.5 (8 kiB)
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The receive buffers is proportional to the number of connections we make, and is
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limited by the total number of connections in the session (default is 200).
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The send buffers is proportional to the number of upload slots that are allowed
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in the session. The default is auto configured based on the observed upload rate.
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The read and write cache can be controlled (see section below).
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The "hash temp" entry size depends on whether or not hashing is optimized for
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speed or memory usage. In this test run it was optimized for memory usage.
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disable disk cache
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------------------
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The bulk of the memory libtorrent will use is used for the disk cache. To save
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the absolute most amount of memory, you can disable the the cache by setting
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``session_settings::cache_size`` to 0. You might want to consider using the cache
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but just disable caching read operations. You do this by settings
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``session_settings::use_read_cache`` to false. This is the main factor in how much
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memory will be used by the client. Keep in mind that you will degrade performance
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by disabling the cache. You should benchmark the disk access in order to make an
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informed trade-off.
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remove torrents
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---------------
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Torrents that have been added to libtorrent will inevitably use up memory, event
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when it's paused. A paused torrent will not use any peer connection objects or
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any send or receive buffers though. Any added torrent holds the entire .torrent
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file in memory, it also remembers the entire list of peers that it's heard about
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(which can be fairly long unless it's capped). It also retains information about
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which blocks and pieces we have on disk, which can be significant for torrents
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with many pieces.
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If you need to minimize the memory footprint, consider removing torrents from
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the session rather than pausing them. This will likely only make a difference
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when you have a very large number of torrents in a session.
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The downside of removing them is that they will no longer be auto-managed. Paused
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auto managed torrents are scraped periodically, to determine which torrents are
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in the greatest need of seeding, and libtorrent will prioritize to seed those.
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socket buffer sizes
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-------------------
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You can make libtorrent explicitly set the kernel buffer sizes of all its peer
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sockets. If you set this to a low number, you may see reduced throughput, especially
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for high latency connections. It is however an oportunity to save memory per
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connection, and might be worth considering if you have a very large number of
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peer connections. This memory will not be visible in your process, this sets
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the amount of kernel memory is used for your sockets.
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Change this by setting ``session_settings::recv_socket_buffer_size`` and
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``session_settings::send_socket_buffer_size``.
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peer list size
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--------------
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The default maximum for the peer list is 4000 peers. For IPv4 peers, each peer
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entry uses 32 bytes, which ends up using 128 kB per torrent. If seeding 4 popular
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torrents, the peer lists alone uses about half a megabyte.
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The default limit is the same for paused torrents as well, so if you have a
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large number of paused torrents (that are popular) it will be even more
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significant.
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If you're short of memory, you should consider lowering the limit. 500 is probably
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enough. You can do this by setting ``session_settings::max_peerlist_size`` to
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the max number of peers you want in the torrent's peer list.
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You should also lower the same limit but for paused torrents. It might even make sense
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to set that even lower, since you only need a few peers to start up while waiting
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for the tracker and DHT to give you fresh ones. The max peer list size for paused
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torrents is set by ``session_settings::max_paused_peerlist_size``.
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The drawback of lowering this number is that if you end up in a position where
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the tracker is down for an extended period of time, your only hope of finding live
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peers is to go through your list of all peers you've ever seen. Having a large
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peer list will also help increase performance when starting up, since the torrent
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can start connecting to peers in parallel with connecting to the tracker.
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send buffer watermark
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---------------------
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The send buffer watermark controls when libtorrent will ask the disk I/O thread
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to read blocks from disk, and append it to a peer's send buffer.
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When the send buffer has fewer than or equal number of bytes as
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``session_settings::send_buffer_watermark``, the peer will ask the disk I/O thread
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for more data to send. The trade-off here is between wasting memory by having too
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much data in the send buffer, and hurting send rate by starving out the socket,
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waiting for the disk read operation to complete.
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If your main objective is memory usage and you're not concerned about being able
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to achieve high send rates, you can set the watermark to 9 bytes. This will guarantee
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that no more than a single (16 kiB) block will be on the send buffer at a time, for
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all peers. This is the least amount of memory possible for the send buffer.
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You should benchmark your max send rate when adjusting this setting. If you have
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a very fast disk, you are less likely see a performance hit.
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optimize hashing for memory usage
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---------------------------------
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When libtorrent is doing hash checks of a file, or when it re-reads a piece that
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was just completed to verify its hash, there are two options. The default one
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is optimized for speed, which allocates buffers for the entire piece, reads in
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the whole piece in one read call, then hashes it.
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The second option is to optimize for memory usage instead, where a single buffer
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is allocated, and the piece is read one block at a time, hashing it as each
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block is read from the file. For low memory environments, this latter approach
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is recommended. Change this by settings ``session_settings::optimize_hashing_for_speed``
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to false. This will significantly reduce peak memory usage, especially for
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torrents with very large pieces.
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reduce executable size
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----------------------
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Compilers generally add a significant number of bytes to executables that make use
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of C++ exceptions. By disabling exceptions (-fno-exceptions on GCC), you can
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reduce the executable size with up to 45%.
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Also make sure to optimize for size when compiling.
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play nice with the disk
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=======================
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When checking a torrent, libtorrent will try to read as fast as possible from the disk.
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The only thing that might hold it back is a CPU that is slow at calculating SHA-1 hashes,
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but typically the file checking is limited by disk read speed. Most operating systems
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today do not prioritize disk access based on the importance of the operation, this means
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that checking a torrent might delay other disk accesses, such as virtual memory swapping
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or just loading file by other (interactive) applications.
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In order to play nicer with the disk, and leave some spare time for it to service other
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processes that might be of higher importance to the end-user, you can introduce a sleep
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between the disc accesses. This is a direct tradeoff between how fast you can check a
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torrent and how soft you will hit the disk.
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You control this by setting the ``session_settings::file_checks_delay_per_block`` to greater
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than zero. This number is the number of milliseconds to sleep between each read of 16 kiB.
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The sleeps are not necessarily in between each 16 kiB block (it might be read in larger chunks),
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but the number will be multiplied by the number of blocks that were read, to maintain the
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same semantics.
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benchmarking
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============
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disk cache stats
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----------------
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disk access stats
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-----------------
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session stats
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-------------
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