I have some questions regarding the optimal entry size setting for Redis hash sets.
In this example memory-optimization they use 100 hash entries
per key but use hash-max-zipmap-entries 256 ? Why not
hash-max-zipmap-entries 100 or 128?
On the redis website (above link) they used max hash entry size of
100, but in this post instagram, they mention 1000 entries. So
does this mean the optimal setting is a function of the product of
hash-max-zipmap-entries & hash-max-zipmap-value ?(ie in this case
Instagram has smaller hash-values than memory optimization example?)
Your comments/clarifications are much appreciated.
The key is, from here:
manipulating the compact versions of these [ziplist] structures can become slow as they grow longer
and
[as ziplists grow longer] fetching/updating individual fields of a HASH, Redis will have to decode many individual entries, and CPU caches won’t be as effective
So to your questions
This page just shows an example and I doubt the author gave much thought to the exact values. In real life, IF you wanted to take advantage of ziplists, and you knew your number of entries per hash was <100, then setting it at 100, 128 or 256 would make no difference. hash-max-zipmap-entries is only the LIMIT over which you're telling Redis to change the encoding from ziplist to hash.
There may be some truth in your "product of hash-max-zipmap-entries & hash-max-zipmap-value" idea, but I'm speculating. More importantly, first you have to define "optimal" based on what you want to do. If you want to do lots of HSET/HGETs in a large ziplist, it will be slower than if you used a hash. But if you never get/update single fields only ever do HMSET/HGETALL on a key, large ziplists wouldn't slow you down. The Instagram 1000 was THEIR optimal number based on THEIR specific data, use cases, and Redis function call frequencies.
You encouraged me to read both links and it seems that you are asking for "default value for hash table size".
I don't think that it's possible to say that one number is universal for all possibilities. The described mechanism is similar to standard hash mapping. Look at http://en.wikipedia.org/wiki/Hash_table
If you have small size of hash-table, it means that many various hash values point into the same array, where the equals method is used to find out the item.
On the other hand, large hash table means that it allocates large memory along with many empty fields. But this scales well as the algorithm uses O(1) big O notation and there is no equals searching for the item.
In general the size of the table IMHO depends on the overall count of all elements you expect to put into the table and it also depends on the diversity of the key. I mean if every hash start with "0001" not even size=100000 would help you.
Related
I have a Redis Master and 2 slaves. All 3 are currently on the same unix server. The memory used by the 3 instances is approximately 3.5 G , 3 G , 3G. There are about 275000 keys in the redis db. About 4000 are hashes. 1 Set has 100000 values. 1 List has 275000 keys in it. Its a List of Hashes and Sets. The server has total memory of 16 GB. Currently 9.5 GB is used. The persistence is currently off. The rdb file is written once in a day by forced background save. Please provide any suggestions for optimizations. max-ziplist configuration is default currently.
Optimizing Hashes
First, let's look at the hashes. Two important questions - how many elements in each hash, and what is the largest value in those hashes? A hash uses the memory efficient ziplist representation if the following condition is met:
len(hash) < hash-max-ziplist-entries && length-of-largest-field(hash) < hash-max-ziplist-value
You should increase the two settings in redis.conf based on your data, but don't increase it more than 3-4 times the default.
Optimizing Sets
A set with 100000 cannot be optimized, unless you provide additional details on your use case. Some general strategies though -
Maybe use HyperLogLog - Are you using the set to count unique elements? If the only commands you run are sadd and scard - maybe you should switch to a hyperloglog.
Maybe use Bloom Filter - Are you using the set to check for existence of a member? If the only commands you run are sadd and sismember - maybe you should implement a bloom filter and use it instead of the set.
How big is each element? - Set members should be small. If you are storing big objects, you are perhaps doing something incorrect.
Optimizing Lists
A single list with 275000 seems wrong. It is going to be slow to access elements in the center of the list. Are you sure you list is the right data structure for your use case?
Change list-compress-depth to 1 or higher. Read about this setting in redis.conf - there are tradeoffs. But for a list of 275000 elements, you certainly want to enable compression.
Tools
Use the open source redis-rdb-tools to analyze your data set (disclaimer: I am the author of this tool). It will tell you how much memory each key is taking. It will help you to decide where to concentrate your efforts on.
You can also refer to this memory optimization cheat sheet.
What else?
You have provided very little details on your use case. The best savings come from picking the right data structure for your use case. I'd encourage you to update your question with more details on what you are storing within the hash / list / set.
We did following configuration and that helped to reduce the memory footprint by 40%
list-max-ziplist-entries 2048
list-max-ziplist-value 10000
list-compress-depth 1
set-max-intset-entries 2048
hash-max-ziplist-entries 2048
hash-max-ziplist-value 10000
Also, we increased the RAM on the linux server and that helped us with the Redis memory issues.
#pozs... this is NOT a duplicate of the one you indicated. That was the first place I looked. I could care less about the difference between text and varchar. I'm asking about physical space used within the medium aka server hard drive.
I know that hard drives are split into blocks of bytes aka chunks, that if used less then the total amount of the block the remaining space is an empty waste of unused space. What I'm curious about is that the text option itself uses a certain amount of storage. Can the space used be reduced rather than just limiting quantity of input. I could say text limit => 1, and it may still use thousands upon thousands of bytes... this is what I'm asking about.
This is a photo of hard drive blocks. This is how I imagine ActiveRecord text type space used
Here's the wiki on Blocks(data storage) http://en.wikipedia.org/wiki/Block_(data_storage) As you can see they say "Block storage is normally abstracted by a file system or database management system (DBMS)" What they do NOT say is HOW it is abstracted.
According to Igor's blog he says "To my surprise, they determined that the average I/O size of our Postgres databases was much higher that 8KB block size, and up to 1MB." http://igorsf.wordpress.com/2010/11/01/things-to-check-when-configuring-new-postgres-storage-for-high-performance-and-availability/ While this is helpful to know it doesn't tell me the default behaviour between ActiveRecord and PostgreSQL in handling blocks.
According to concernedoftunbridgewells "The database will allocate space in a table or index in some given block size. In the case of Postgres this is 8K". https://dba.stackexchange.com/questions/15510/understanding-block-sizes/15514#15514?newreg=fc10593601be479b8ed697d1bbd108ed So if 8K is used as a block, then how high or low do I set the text type limit to match and fit within the one 8K block, because it may use more then just one block.
I know that PostgreSQL block size setting can be changed. So I would like clarity on "how ActiveRecord PostgreSQL block size handling currently works". I will accept a good answer for that.
A page contains more than one item (assuming there is space obviously). If your row is less than 8k, then other rows will be stored on the same page with it (I simplify slightly - postgres stores large columns separately anyway). Limiting the max length of a column doesn't interact with this.
My reading of the details on character types is that strings under 126 characters incur 3 bytes less overhead, but this happens on a row by row basis, independently of what the maximum length is.
The postgresql docs have details on the exact on disk format and how postgresql deals with large columns.
IMO, the size takes by a text type db column is mainly depends on the content store in the column. The limit setting inside the ActiveRecord will just do a validation before the content is saving into the db column, and didn't has an impact to the actual storage.
I would like to store millions of data lines that looks like this:
key, value
key is an integer in the range of (0 to 5,000,000); all values are unique;
value is an unsigned int16 value (0 to 65535)
the key is to store the data while taking the LEAST AMOUNT OF DISK SPACE, and yet, be able to query the data. can you think of any algorithms / smart schemes for data storage that would be helpful?
just in case it matters, I use Linux.
One option would be, if the key values are not important data but rather just index data to utilize a flat file of bits ( with a descriptive header ). Every 16 bits is a value and the nth value would then be (n - 1) * 16 bits from the end of the header.
Additionally, if the key value does matter, a set flat file of about 10MB would allow for the entire key space to be stored without storing actual keys. The 16 bits that are at the (n - 1) * 16 offset would be that key's value.
That would probably be the least space intensive method for storage, as it would be only the data that is literally required. ( Though, if you are only interested in say 100k values and one has a key of 5 million you do end up with a lot of wasted space, which wouldn't be there with an actual key,value addressing system. So this methodology only achieves a minimum disk storage for sets of tightly grouped values or many many numbers (over about the 2 million mark ).
how do you plan to use stored data? with random or sequential access? for sequential access you can use any archiving algorithm, e.g. LZMA. Random access doesn't leave you a lot of space for improvements.
can you see any patterns of this data? e.g. if the difference between adjacent keys/values are often small you can store only packed differences. and million of other possible approaches.
[EDIT] also you can check techniques used for data compression in network communication
[EDIT1] and you can check this Google Code Integer Array Compression project
This depend upon the operation and data. I would first recommend "just using a database" (a simple key-value store such as BDB/EhCache [read: Key Value store], for instance :-)
Mimisbrunnr also has a good answer if all the keys are used.
If the keys are near constant/read-only and only a relatively small percent of the keys are used, consider the use of a (disk-based) Heap data-structure (very similar to an Array-based Heap; Heaps need not be Array-based). Robert Sedgewick had a good book from the late 80's that had a very lean implementation, but I forget the name. A Heap will be more beneficial when compared to a flat index with a smaller proportion of used keys and at full-load will have worse storage requirements.
(If abstracted, the used method could be switched and/or a hybrid heap with indexed/sequenced leaf-node values could be used [along with Huffman encoding or whatnot], but that is just adding far more complications. Keep it simple ... hence first suggestion of an existing key/value store ;-)
Happy coding.
Have you considered using a database designed for mobile devices such as SQL Server Compact, or another similar database? These will have a small footprint on the disk, while still providing the full search power you need.
Another example of a compact database engine is KeyDB for linux:
http://3d2f.com/programs/11-989-keydb-download.shtml
I have a choice.
I have a number of already ordered strings that I need to store and access. It looks like I can choose between using:
A TStringList
A Dynamic Array of strings, and
A Linked List of strings (singly linked)
and Alan in his comment suggested I also add to the choices:
TList<string>
In what circumstances is each of these better than the others?
Which is best for small lists (under 10 items)?
Which is best for large lists (over 1000 items)?
Which is best for huge lists (over 1,000,000 items)?
Which is best to minimize memory use?
Which is best to minimize loading time to add extra items on the end?
Which is best to minimize access time for accessing the entire list from first to last?
On this basis (or any others), which data structure would be preferable?
For reference, I am using Delphi 2009.
Dimitry in a comment said:
Describe your task and data access pattern, then it will be possible to give you an exact answer
Okay. I've got a genealogy program with lots of data.
For each person I have a number of events and attributes. I am storing them as short text strings but there are many of them for each person, ranging from 0 to a few hundred. And I've got thousands of people. I don't need random access to them. I only need them associated as a number of strings in a known order attached to each person. This is my case of thousands of "small lists". They take time to load and use memory, and take time to access if I need them all (e.g. to export the entire generated report).
Then I have a few larger lists, e.g. all the names of the sections of my "virtual" treeview, which can have hundreds of thousands of names. Again I only need a list that I can access by index. These are stored separately from the treeview for efficiency, and the treeview retrieves them only as needed. This takes a while to load and is very expensive memory-wise for my program. But I don't have to worry about access time, because only a few are accessed at a time.
Hopefully this gives you an idea of what I'm trying to accomplish.
p.s. I've posted a lot of questions about optimizing Delphi here at StackOverflow. My program reads 25 MB files with 100,000 people and creates data structures and a report and treeview for them in 8 seconds but uses 175 MB of RAM to do so. I'm working to reduce that because I'm aiming to load files with several million people in 32-bit Windows.
I've just found some excellent suggestions for optimizing a TList at this StackOverflow question:
Is there a faster TList implementation?
Unless you have special needs, a TStringList is hard to beat because it provides the TStrings interface that many components can use directly. With TStringList.Sorted := True, binary search will be used which means that search will be very quick. You also get object mapping for free, each item can also be associated with a pointer, and you get all the existing methods for marshalling, stream interfaces, comma-text, delimited-text, and so on.
On the other hand, for special needs purposes, if you need to do many inserts and deletions, then something more approaching a linked list would be better. But then search becomes slower, and it is a rare collection of strings indeed that never needs searching. In such situations, some type of hash is often used where a hash is created out of, say, the first 2 bytes of a string (preallocate an array with length 65536, and the first 2 bytes of a string is converted directly into a hash index within that range), and then at that hash location, a linked list is stored with each item key consisting of the remaining bytes in the strings (to save space---the hash index already contains the first two bytes). Then, the initial hash lookup is O(1), and the subsequent insertions and deletions are linked-list-fast. This is a trade-off that can be manipulated, and the levers should be clear.
A TStringList. Pros: has extended functionality, allowing to dynamically grow, sort, save, load, search, etc. Cons: on large amount of access to the items by the index, Strings[Index] is introducing sensible performance lost (few percents), comparing to access to an array, memory overhead for each item cell.
A Dynamic Array of strings. Pros: combines ability to dynamically grow, as a TStrings, with the fastest access by the index, minimal memory usage from others. Cons: limited standard "string list" functionality.
A Linked List of strings (singly linked). Pros: the linear speed of addition of an item to the list end. Cons: slowest access by the index and searching, limited standard "string list" functionality, memory overhead for "next item" pointer, spead overhead for each item memory allocation.
TList< string >. As above.
TStringBuilder. I does not have a good idea, how to use TStringBuilder as a storage for multiple strings.
Actually, there are much more approaches:
linked list of dynamic arrays
hash tables
databases
binary trees
etc
The best approach will depend on the task.
Which is best for small lists (under
10 items)?
Anyone, may be even static array with total items count variable.
Which is best for large lists (over 1000 items)?
Which is best for huge lists (over 1,000,000 items)?
For large lists I will choose:
- dynamic array, if I need a lot of access by the index or search for specific item
- hash table, if I need to search by the key
- linked list of dynamic arrays, if I need many item appends and no access by the index
Which is best to minimize memory use?
dynamic array will eat less memory. But the question is not about overhead, but about on which number of items this overhead become sensible. And then how to properly handle this number of items.
Which is best to minimize loading time to add extra items on the end?
dynamic array may dynamically grow, but on really large number of items, memory manager may not found a continous memory area. While linked list will work until there is a memory for at least a cell, but for cost of memory allocation for each item. The mixed approach - linked list of dynamic arrays should work.
Which is best to minimize access time for accessing the entire list from first to last?
dynamic array.
On this basis (or any others), which data structure would be preferable?
For which task ?
If your stated goal is to improve your program to the point that it can load genealogy files with millions of persons in it, then deciding between the four data structures in your question isn't really going to get you there.
Do the math - you are currently loading a 25 MB file with about 100000 persons in it, which causes your application to consume 175 MB of memory. If you wish to load files with several millions of persons in it you can estimate that without drastic changes to your program you will need to multiply your memory needs by n * 10 as well. There's no way to do that in a 32 bit process while keeping everything in memory the way you currently do.
You basically have two options:
Not keeping everything in memory at once, instead using a database, or a file-based solution which you load data from when you need it. I remember you had other questions about this already, and probably decided against it, so I'll leave it at that.
Keep everything in memory, but in the most space-efficient way possible. As long as there is no 64 bit Delphi this should allow for a few million persons, depending on how much data there will be for each person. Recompiling this for 64 bit will do away with that limit as well.
If you go for the second option then you need to minimize memory consumption much more aggressively:
Use string interning. Every loaded data element in your program that contains the same data but is contained in different strings is basically wasted memory. I understand that your program is a viewer, not an editor, so you can probably get away with only ever adding strings to your pool of interned strings. Doing string interning with millions of string is still difficult, the "Optimizing Memory Consumption with String Pools" blog postings on the SmartInspect blog may give you some good ideas. These guys deal regularly with huge data files and had to make it work with the same constraints you are facing.
This should also connect this answer to your question - if you use string interning you would not need to keep lists of strings in your data structures, but lists of string pool indexes.
It may also be beneficial to use multiple string pools, like one for names, but a different one for locations like cities or countries. This should speed up insertion into the pools.
Use the string encoding that gives the smallest in-memory representation. Storing everything as a native Windows Unicode string will probably consume much more space than storing strings in UTF-8, unless you deal regularly with strings that contain mostly characters which need three or more bytes in the UTF-8 encoding.
Due to the necessary character set conversion your program will need more CPU cycles for displaying strings, but with that amount of data it's a worthy trade-off, as memory access will be the bottleneck, and smaller data size helps with decreasing memory access load.
One question: How do you query: do you match the strings or query on an ID or position in the list?
Best for small # strings:
Whatever makes your program easy to understand. Program readability is very important and you should only sacrifice it in real hotspots in your application for speed.
Best for memory (if that is the largest constrained) and load times:
Keep all strings in a single memory buffer (or memory mapped file) and only keep pointers to the strings (or offsets). Whenever you need a string you can clip-out a string using two pointers and return it as a Delphi string. This way you avoid the overhead of the string structure itself (refcount, length int, codepage int and the memory manager structures for each string allocation.
This only works fine if the strings are static and don't change.
TList, TList<>, array of string and the solution above have a "list" overhead of one pointer per string. A linked list has an overhead of at least 2 pointers (single linked list) or 3 pointers (double linked list). The linked list solution does not have fast random access but allows for O(1) resizes where trhe other options have O(lgN) (using a factor for resize) or O(N) using a fixed resize.
What I would do:
If < 1000 items and performance is not utmost important: use TStringList or a dyn array whatever is easiest for you.
else if static: use the trick above. This will give you O(lgN) query time, least used memory and very fast load times (just gulp it in or use a memory mapped file)
All mentioned structures in your question will fail when using large amounts of data 1M+ strings that needs to be dynamically chaned in code. At that Time I would use a balances binary tree or a hash table depending on the type of queries I need to maken.
From your description, I'm not entirely sure if it could fit in your design but one way you could improve on memory usage without suffering a huge performance penalty is by using a trie.
Advantages relative to binary search tree
The following are the main advantages
of tries over binary search trees
(BSTs):
Looking up keys is faster. Looking up a key of length m takes worst case
O(m) time. A BST performs O(log(n))
comparisons of keys, where n is the
number of elements in the tree,
because lookups depend on the depth of
the tree, which is logarithmic in the
number of keys if the tree is
balanced. Hence in the worst case, a
BST takes O(m log n) time. Moreover,
in the worst case log(n) will approach
m. Also, the simple operations tries
use during lookup, such as array
indexing using a character, are fast
on real machines.
Tries can require less space when they contain a large number of short
strings, because the keys are not
stored explicitly and nodes are shared
between keys with common initial
subsequences.
Tries facilitate longest-prefix matching, helping to find the key
sharing the longest possible prefix of
characters all unique.
Possible alternative:
I've recently discovered SynBigTable (http://blog.synopse.info/post/2010/03/16/Synopse-Big-Table) which has a TSynBigTableString class for storing large amounts of data using a string index.
Very simple, single layer bigtable implementation, and it mainly uses disc storage, to consumes a lot less memory than expected when storing hundreds of thousands of records.
As simple as:
aId := UTF8String(Format('%s.%s', [name, surname]));
bigtable.Add(data, aId)
and
bigtable.Get(aId, data)
One catch, indexes must be unique, and the cost of update is a bit high (first delete, then re-insert)
TStringList stores an array of pointer to (string, TObject) records.
TList stores an array of pointers.
TStringBuilder cannot store a collection of strings. It is similar to .NET's StringBuilder and should only be used to concatenate (many) strings.
Resizing dynamic arrays is slow, so do not even consider it as an option.
I would use Delphi's generic TList<string> in all your scenarios. It stores an array of strings (not string pointers). It should have faster access in all cases due to no (un)boxing.
You may be able to find or implement a slightly better linked-list solution if you only want sequential access. See Delphi Algorithms and Data Structures.
Delphi promotes its TList and TList<>. The internal array implementation is highly optimized and I have never experienced performance/memory issues when using it. See Efficiency of TList and TStringList
I have an application in which I have to store a couple of millions of integers, I have to store them in a Look up table, obviously I cannot store such amount of data in memory and in my requirements I am very limited I have to store the data in an embebedded system so I am very limited in the space, so I would like to ask you about recommended methods that I can use for the reduction of the look up table. I cannot use function approximation such as neural networks, the values needs to be in a table. The range of the integers is not known at the moment. When I say integers I mean a 32 bit value.
Basically the idea is use some copmpression method to reduce the amount of memory but without losing many precision. This thing needs to run in hardware so the computation overhead cannot be very high.
In my algorithm I have to access to one value of the table do some operations with it and after update the value. In the end what I should have is a function which I pass an index to it and then I get a value, and after I have to use another function to write a value in the table.
I found one called tile coding , this one is based on several look up tables, does anyone know any other method?.
Thanks.
I'd look at the types of numbers you need to store and pull out the information that's common for many of them. For example, if they're tightly clustered, you can take the mean, store it, and store the offsets. The offsets will have fewer bits than the original numbers. Or, if they're more or less uniformly distributed, you can store the first number and then store the offset to the next number.
It would help to know what your key is to look up the numbers.
I need more detail on the problem. If you cannot store the real value of the integers but instead an approximation, that means you are going to reduce (throw away) some of the data (detail), correct? I think you are looking for a hash, which can be an artform in itself. For example say you have 32 bit values, one hash would be to take the 4 bytes and xor them together, this would result in a single 8 bit value, reducing your storage by a factor of 4 but also reducing the real value of original data. Typically you could/would go further and perhaps and only use a few of those 8 bits , say the lower 4 and reduce the value further.
I think my real problem is either you need the data or you dont, if you need the data you need to compress it or find more memory to store it. If you dont, then use a hash of some sort to reduce the number of bits until you reach the amount of memory you have for storage.
Read http://www.cs.ualberta.ca/~sutton/RL-FAQ.html
"Function approximation" refers to the
use of a parameterized functional form
to represent the value function
(and/or the policy), as opposed to a
simple table."
Perhaps that applies. Also, update your question with additional facts -- don't merely answer in the comments.
Edit.
A bit array can easily store a bit for each of your millions of numbers. Let's say you have numbers in the range of 1 to 8 million. In a single megabyte of storage you can have a 1 bit for each number in your set and a 0 for each number not in your set.
If you have numbers in the range of 1 to 32 million, you'll require 4Mb of memory for a big table of all 32M distinct numbers.
See my answer to Modern, high performance bloom filter in Python? for a Python implementation of a bit array of unlimited size.
If you are merely looking for the presence of the number in question a bloom filter, might be what you are looking for. Honestly though your question is fairly vague and confusing. It would help to explain what Q values are, and what you do with them once you find them in the table.
If your set of integers is homongenous, then you could try a hash table, because there is a trick you can use to cut the size of the stored integers, in your case, in half.
Assume the integer, n, because its set is homogenous can be the hash. Assume you have 0x10000 (16k) buckets. Each bucket index, iBucket = n&FFFF. Each item in a bucket need only store 16 bits, since the first 16 bits are the bucket index. The other thing you have to do to keep the data small is to put the count of items in the bucket, and use an array to hold the items in the bucket. Using a linked list will be too large and slow. When you iterate the array looking for a match, remember you only need to compare the 16 bits that are stored.
So assuming a bucket is a pointer to the array and a count. On a 32 bit system, this is 64 bits max. If the number of ints was small enough we might be able to do some fancy things and use 32 bits for a bucket. 16k * 8 bytes = 524k, 2 million shorts = 4mb. So this gets you a method to lookup the ints and about 40% compression.