add more details about resource usage

docs/internals.rst

@@ -364,7 +364,7 @@ varies between 33% and 300%.
 Indexes / Caches memory usage
 -----------------------------
 
-Here is the estimated memory usage of |project_name|:
+Here is the estimated memory usage of |project_name| - it's complicated:
 
   chunk_count ~= total_file_size / 2 ^ HASH_MASK_BITS
 
@@ -377,6 +377,14 @@ Here is the estimated memory usage of |project_name|:
   mem_usage ~= repo_index_usage + chunks_cache_usage + files_cache_usage
              = chunk_count * 164 + total_file_count * 240
 
+Due to the hashtables, the best/usual/worst cases for memory allocation can
+be estimated like that:
+
+  mem_allocation = mem_usage / load_factor  # l_f = 0.25 .. 0.75
+
+  mem_allocation_peak = mem_allocation * (1 + growth_factor)  # g_f = 1.1 .. 2
+
+
 All units are Bytes.
 
 It is assuming every chunk is referenced exactly once (if you have a lot of
@@ -388,6 +396,17 @@ more chunks than estimated above, because 1 file is at least 1 chunk).
 
 If a remote repository is used the repo index will be allocated on the remote side.
 
+The chunks cache, files cache and the repo index are all implemented as hash
+tables. A hash table must have a significant amount of unused entries to be
+fast - the so-called load factor gives the used/unused elements ratio.
+
+When a hash table gets full (load factor getting too high), it needs to be
+grown (allocate new, bigger hash table, copy all elements over to it, free old
+hash table) - this will lead to short-time peaks in memory usage each time this
+happens. Usually does not happen for all hashtables at the same time, though.
+For small hash tables, we start with a growth factor of 2, which comes down to
+~1.1x for big hash tables.
+
 E.g. backing up a total count of 1 Mi (IEC binary prefix i.e. 2^20) files with a total size of 1TiB.
 
 a) with ``create --chunker-params 10,23,16,4095`` (custom, like borg < 1.0 or attic):

docs/usage.rst

@@ -206,36 +206,79 @@ Resource Usage
 
 |project_name| might use a lot of resources depending on the size of the data set it is dealing with.
 
-CPU:
+If one uses |project_name| in a client/server way (with a ssh: repository),
+the resource usage occurs in part on the client and in another part on the
+server.
+
+If one uses |project_name| as a single process (with a filesystem repo),
+all the resource usage occurs in that one process, so just add up client +
+server to get the approximate resource usage.
+
+CPU client:
+    borg create: does chunking, hashing, compression, crypto (high CPU usage)
+    chunks cache sync: quite heavy on CPU, doing lots of hashtable operations.
+    borg extract: crypto, decompression (medium to high CPU usage)
+    borg check: similar to extract, but depends on options given.
+    borg prune / borg delete archive: low to medium CPU usage
+    borg delete repo: done on the server
     It won't go beyond 100% of 1 core as the code is currently single-threaded.
     Especially higher zlib and lzma compression levels use significant amounts
-    of CPU cycles.
+    of CPU cycles. Crypto might be cheap on the CPU (if hardware accelerated) or
+    expensive (if not).
 
-Memory (RAM):
+CPU server:
+    It usually doesn't need much CPU, it just deals with the key/value store
+    (repository) and uses the repository index for that.
+
+    borg check: the repository check computes the checksums of all chunks
+    (medium CPU usage)
+    borg delete repo: low CPU usage
+
+CPU (only for client/server operation):
+    When using borg in a client/server way with a ssh:-type repo, the ssh
+    processes used for the transport layer will need some CPU on the client and
+    on the server due to the crypto they are doing - esp. if you are pumping
+    big amounts of data.
+
+Memory (RAM) client:
     The chunks index and the files index are read into memory for performance
-    reasons.
+    reasons. Might need big amounts of memory (see below).
     Compression, esp. lzma compression with high levels might need substantial
     amounts of memory.
 
-Temporary files:
-    Reading data and metadata from a FUSE mounted repository will consume about
-    the same space as the deduplicated chunks used to represent them in the
-    repository.
-
-Cache files:
-    Contains the chunks index and files index (plus a compressed collection of
-    single-archive chunk indexes).
+Memory (RAM) server:
+    The server process will load the repository index into memory. Might need
+    considerable amounts of memory, but less than on the client (see below).
 
-Chunks index:
+Chunks index (client only):
     Proportional to the amount of data chunks in your repo. Lots of chunks
     in your repo imply a big chunks index.
     It is possible to tweak the chunker params (see create options).
 
-Files index:
-    Proportional to the amount of files in your last backup. Can be switched
+Files index (client only):
+    Proportional to the amount of files in your last backups. Can be switched
     off (see create options), but next backup will be much slower if you do.
 
-Network:
+Repository index (server only):
+    Proportional to the amount of data chunks in your repo. Lots of chunks
+    in your repo imply a big repository index.
+    It is possible to tweak the chunker params (see create options) to
+    influence the amount of chunks being created.
+
+Temporary files (client):
+    Reading data and metadata from a FUSE mounted repository will consume about
+    the same space as the deduplicated chunks used to represent them in the
+    repository.
+
+Temporary files (server):
+    Not much.
+
+Cache files (client only):
+    Contains the chunks index and files index (plus a collection of single-
+    archive chunk indexes which might need huge amounts of disk space,
+    depending on archive count and size - see FAQ about how to reduce).
+
+Network (only for client/server operation):
     If your repository is remote, all deduplicated (and optionally compressed/
     encrypted) data of course has to go over the connection (ssh: repo url).
     If you use a locally mounted network filesystem, additionally some copy
@@ -243,7 +286,8 @@ Network:
     you backup multiple sources to one target repository, additional traffic
     happens for cache resynchronization.
 
-In case you are interested in more details, please read the internals documentation.
+In case you are interested in more details (like formulas), please read the
+internals documentation.
 
 
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