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Stateless ZIP library - SLZ


SLZ is a fast and memory-less stream compressor which produces an output that can be decompressed with zlib or gzip. It does not implement decompression at all, zlib is perfectly fine for this.

The purpose is to use SLZ in situations where a zlib-compatible stream is needed and zlib's resource usage would be too high while the compression ratio is not critical. The typical use case is in HTTP servers and gateways which have to compress many streams in parallel with little CPU resources to assign to this task, and without having to thottle the compression ratio due to the memory usage. In such an environment, the server's memory usage can easily be divided by 10 and the CPU usage by 3.

How does it work ?

While zlib uses 256 kB of memory per stream in addition to a few tens of bytes for the stream descriptor itself, SLZ only stores a stream descriptor made of 28 bytes. Thus it is particularly suited to environments having to deal with tens to hundreds of thousands of concurrent streams.

The key difference between zlib and SLZ is that SLZ is stateless in that it doesn't consider the data previously compressed as part of its dictionary. It doesn't hurt compression performance when it is fed in large enough chunks (at least a few kB at once), but it must not be used if it has to read less than a few kB at a time, or the caller will have to implement the input buffer to maximize the compression ratio. For example, an HTTP gateway may defragment all the parts from a chunked-encoded response buffer into a linear buffer that is then fed at once to SLZ. This temporary buffer would then be static and shared between all streams running on the same thread.

A side effect from its stateless nature is that it can emit a zlib-compatible stream without being vulnerable to CRIME-like attacks by processing the sensitive and attacker-controlled data in distinct batches. Since no dictionary persists between calls, there is no risk that attacker-controlled data is used to guess sensitive data using dictionary lookups.

Another induced benefit which was not initially seeked is that SLZ is on average 3 times as fast as zlib in its fastest mode. This is the result of the simpler matching and encoding mechanisms involved which permit even more optimisations.

Reference tests

Some performance tests were run on various types of data showing anything between 227 MB/s and 753 MB/s per core on the test machine depending on the difficulty to find matching strings and on the amount of data emitted as literals (hence not compressed). In the end, comparative tests were performed using some reference datasets that are already being used for other libraries. This performance report was produced using the Silesia Corpus to get something comparable to what is reported by the LZ4 utility here :

The tests were run on a single core of a core i5-3320M at 3.3 GHz (turbo mode).

    ProductInput BWCompression ratioNotes
    lz4-r124 352 MB/s2.09
    lzo-2.09 347 MB/s2.10
    slz-142 266 MB/s2.12deflate format
    slz-142 214 MB/s2.12gzip format
    zlib-1.28 -1 -h 88 MB/s1.64huffman only
    zlib-1.28 -1 -r 81 MB/s1.65RLE only
    zlib-1.28 -1 -F 70 MB/s2.32fixed huffman
    zlib-1.28 -1 70 MB/s2.74

Only zlib and SLZ can emit a stream that a regular zlib-compatible browser can decompress (RFC1950/1951/1952 aka zlib, deflate, gzip). Here SLZ is 3 times faster than zlib for a less compressed payload (29% larger than zlib). Tests run on other file types (including HTML, CSS, JS) show that the ratio of compression speed between SLZ and zlib varies from 2.5 to 3.5. The output compressed stream is often 24 to 32% larger for SLZ than zlib.

Since SLZ focuses a lot on performance savings gained by keeping most of the workload in CPU cache, it scales very well when multiple cores are used in parallel (low memory bandwidth) as shown in this tests run on a quad-core ARM Cortex A17 (RK3288) running at 1.8 GHz and the same input data :

    #CoresInput sizeTime(s)BW/core(MB/s)Total BW(MB/s)
    12119385803.57559.3 59.3

Increasing compression ratio in HTTP gateways

Since some products automatically throttle the compression level based on the CPU usage or the available memory, on high speed networks, zlib can end up showing a much lower average compression ratio in order to fill an uplink. With SLZ, no extra memory usage is needed for compression, and the CPU usage is reduced by 2.5-3.5 times, meaning that the throttling will happen at much higher data rates. The net effect is a significant improvement on the compression ratio compared to zlib at a given resource usage.

HAProxy running on a single core of a core i5-3320M, with 500 concurrent users, the default 16kB buffers, and accessing (76 kB) shows the following measures with traffic leaving the machine via a gigabit Ethernet link (hence the 976 Mbps of IP traffic) after being compressed with zlib and SLZ, respectively :

At 100% CPU (no CPU usage limit) :

    LibraryBW InBW OutBW SavedRatiomemory VSZ/RSShits/s
    zlib480 Mbps188 Mbps292 Mbps2.55130M / 101M744
    slz1700 Mbps810 Mbps890 Mbps2.1023.7M / 9.7M2382

At 85% CPU (limited) :

    LibraryBW InBW OutBW SavedRatiomemory VSZ/RSS hits/s
    zlib1240 Mbps976 Mbps264 Mbps1.27130M / 100M1738
    slz1600 Mbps976 Mbps624 Mbps1.6423.7M / 9.7M2210

In the CPU-limited test, once CPU usage limit was reached, blocks were sent uncompressed. This explains how the gigabit link was always saturated, and it is then the input bandwidth which varies according to the average compression ratio. In all situations, SLZ resulted in bandwidth savings 3 times higher than zlib.

No magics inside!

There are 6 key points having a large impact on compression speed in any LZ- based compressor :

  1. dictionary lookup : looking up a sequence of bytes in a recent window is not necessarily fast. SLZ uses a hash-based technique similar to LZO or LZ4, focusing on keeping track of only the most recent occurrence of a given sequence. The hash table contains the absolute position in the stream of the last occurrence of the hashed entry. This table is initialized to impossible values prior to compressing so that unused entries are not even evaluated, as it was found to be faster than not initializing them. During tests, it was found that hashing 4 bytes to one value among 4096-65536 resulted in the best compression ratio, and that 8192 entries was the best tradeoff between speed and compression ratio. Smaller hash tables cause more collisions hurting the compression ratio, and larger ones experience a significant slowdown on certain datasets due to the hashtable having more difficulties to fit entirely in the CPU cache.
  2. indexing : if sequences of less than 3 bytes are considered for a lookup, since we only keep the last occurrence, there's a high risk of not keeping the most useful sequence which has large chances of matching a similar word. For example in HTML a lot of occurrences of "><", "</", "/>" can be found so the probability that the one from "</a>" is picked instead of the one from "</td>" will make it hard to find long matches. Conversely, sequences larger than 4 bytes will only match once in a while and will prevent shorter series of characters from being matched. Experimentation has shown that hashing 3 bytes only provided the same compression ratio as hashing 4 bytes, with up to 10% faster input processing thanks to the reduced cost of hashing. Attempts to index the repeated data were made, and caused a significant performance drop (about 5%) for less than 1% compression ratio improvement, so they were abandonned.
  3. longest match : the longest match measurement is very similar to an strcmp() call except that we count similar characters. In order to avoid this slow operation most of the time, we store in the hash table a copy of the 4 bytes that were hashed. This makes it easy to perform a comparison and to skip those which do not match, then to only start the longest match measurement for parts which already match on the first 4 bytes. With a 13-bits hash on 32-bit character sequence, there's a theorical 1 chance over 524288 that a matching hash implies a matching sequence. But in practice on selected text this chance is significantly higher since not all 32 bits are used, it's more like 24 bits (6 bits per char), so it's one chance over 2048 that an entry will match. That's still 2047/2048 useless calls to the memmatch function that are saved this way.
  4. encoding the output : deflate is often criticized for its bit-oriented stream which is expensive to manipulate as it requires bit shifts. Moreover, on top of this the deflate stream supports several encoding modes, the most common one being dynamic huffman trees, which requires some significant maintenance to keep a valid tree up to date and which SLZ doesn't use. Instead, SLZ uses static huffman trees, and when too many binary entries are found to keep this encoding efficient, it switches to the plain literal mode. Proceeding like this comes with a significant benefit : all code sequences, all distances and all lengths are stored as their respective huffman output bits. This avoids an encoding phase, sequences of bits are immediately sent to the destination buffer without any further operation, and remain reasonably cheap compared to the more common usages.
  5. checksumming : 3 checksum methods exist depending on the encoding format. RFC1950 (zlib format) uses an Adler-32 checksum which theorically involves modulus (divide) but which can be optimized as it's done with a number very close to a power of two. RFC1951 (raw deflate) doesn't involve any checksum. RFC1952 (gzip) uses a CRC-32 which is somewhat expensive but can be optimized in various ways. Since SLZ was designed around gzip mainly, most of the optimizations were done to optimize CRC-32. Some well-known methods involve pre-computed tables, which can be further optimized for multi-byte processing and single-byte processing as well. It was found that updating the CRC-32 on a per-character basis before looking up the hash came at almost no cost because it would probably benefit make use of some pipeline stalls when the hashed entry was not in the CPU's L1 cache. But in order to avoid multiplying the functions, the checksum computation was moved out of the function with a very low extra cost (less than 1%) as it can process all the input at once 4 bytes at a time, contributes to preloading the input data into the caches.
  6. implementation-specific optimizations : modern CPUs can load unaligned words from memory with minimal to no overhead. On architectures that support this (ix86, x86_64, armv7), this is used to limit the number of operations and memory accesses. Some CPUs have a smaller cache and the direct mapping between distances and huffman sequences can make it thrash a lot. Some experimentations were made using a direct mapping only for shortest distances (the most common ones), but results were not encouraging for now as a cache miss is not completely offset by the amount of extra operations.

These two factors have a significant impact on the compression ratio :

  1. collisions in the dictionary, caused by suboptimal hash distribution. Here with 8k entries we have what looks like the best tradeoff between speed and compression ratio for most workloads tested, and it can easily be changed using a macro.
  2. output encoding : the dynamic huffman trees would definitely shorten the output stream but at an important performance cost. But even with fixed trees it is possible to waste a lot of space. First, input bytes 144 to 255 are encoded on 9 bits per byte. SLZ constantly monitors the output stream to decide when it's preferable to switch to plain literals which cost exactly 52 bits to switch back and forth and will not inflate the output stream any further. Second, referencing a match can be longer than the data it replaces. This is true for short sequences, but it can be true even for a long sequence in the middle of a series of plain literals. By monitoring the output stream and verifying the encoding cost before switching, it is possible to avoid any overhead beyond the base 5 bytes per 32kB plus headers for non-compressible data.

Get it!

SLZ is provided as a library with a few extra tools (eg: zenc, a compressor emitting the various formats). It is distributed under the X11 license, meaning that you can do a lot of things with it, such as merge it into GPL or BSD-licensed programs, as well as use it in proprietary software. Contributions are welcome and should be sent as Git patches (see "git format-patch") and will be made under the same license exclusively.

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  • Contact me : Willy TARREAU