The prefetch cache aims to improve the performance of sequential read data.
Of most interest here are the requests of a small size of data for sequential
read, such requests can be optimized by extending them and moving into
the prefetch cache. However, there are 2 issues:
- In aggregate only a small portion of requests is sequential, so delays
caused
by the need to read more volumes of data will lead to an overall decrease
in performance.
- The presence of redundant data in the cache memory with a large number of
random requests.
This pcache implementation solves the above and other problems prefetching data.
The pcache algorithm can be summarised by the following main steps.
1. Monitor I/O requests to identify typical sequences.
This implementation of prefetch cache works at the storage system level and has
information only about the physical block addresses of I/O requests. Statistics
are collected only from read requests to a maximum size of 32kb(by default),
each request that matches the criteria falls into a pool of requests. In order
to store requests statistic used by the rb-tree(lreq.tree), it's simple but for
this issue a quite efficient data structure.
2. Identifying sequential I/O streams.
For each read request to be carried out attempting to lift the chain sequence
from lreq.tree, where this request will be element of a sequential chain of
requests. The key to search for consecutive requests is the area of sectors
preceding the current request. The size of this area should not be too small to
avoid false readahead. The sequential stream data requests can be identified
even when a large number of random requests. For example, if there is access to
the blocks 100, 1157, 27520, 4, 101, 312, 1337, 102, in the context of request
processing 102 will be identified the chain of sequential requests 100, 101. 102
and then should a decision be made to do readahead. Also a situation may arise
when multiple applications A, B, C simultaneously perform sequential read of
data. For each separate application that will be sequential read data
A(100, 101, 102), B(300, 301, 302), C(700, 701, 702), but for block devices it
may look like a random data reading: 100,300,700,101,301,701,102,302,702.
In this case, the sequential streams will also be recognised because location
requests in the rb-tree will allow to separate the sequential I/O streams.
3. Do the readahead into the cache for recognized sequential data streams.
After the issue of the detection of pcache case was resolved, need using larger
requests to bring data into the cache. In this implementation the pcache used
readahead instead of the extension request, therefore the request goes as is.
There is not any reason to put data in the cache that will never be picked up,
but this will always happen in the case of extension requests. In order to store
areas of cached blocks is also used by the rb-tree(pcache.tree), it's simple but
for this issue a quite efficient data structure.
4. Control size of the prefetch cache pool and the requests statistic pool
For control the border of the pool statistic of requests, the data of requests
are placed and replaced according to the FIFO principle, everything is simple.
For control the boundaries of the memory cache used LRU list, it allows to limit
the max amount memory that we can allocate for pcache. But the LRU is there
mainly to prevent displacement of the cache blocks that was read partially.
The main way the memory is pushed out immediately after use, as soon as a chunk
of memory from the cache has been completely read, since the probability of
repetition of the request is very low. Cases when one and the same portion of
the cache memory has been read several times are not optimized and do not apply
to the cases that can optimize the pcache. Thus, using a cache memory of small
volume, by the optimization of the operations read-ahead and clear memory, we
can read entire volumes of data, providing a 100% cache hit. Also does not
decrease the effectiveness of random read requests.
PCache is implemented as a qemu block filter driver, has some configurable
parameters, such as: total cache size, readahead size, maximum size of block
that can be processed.
For performance evaluation has been used several test cases with different
sequential and random read data on SSD disk. Here are the results of tests and
qemu parameters:
qemu parameters:
-M pc-i440fx-2.4 --enable-kvm -smp 4 -m 1024
-drive file=centos7.qcow2,if=none,id=drive-virtio-disk0,format=qcow2,cache=none,
aio=native,pcache-full-size=4MB,pcache-readahead-size=128KB,
pcache-max-aio-size=32KB
-device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x8,drive=drive-virtio-disk0,
id=virtio-disk0
(-set device.virtio-disk0.x-data-plane=on)
********************************************************************************
* Testcase * Results in iops *
* **********************************************
* * clean qemu * pcache * x-data-plane *
********************************************************************************
* Create/open 16 file(s) of total * 25514 req/s * 85659 req/s * 28249 req/s *
* size 2048.00 MB named * 25692 req/s * 89064 req/s * 27950 req/s *
* /tmp/tmp.tmp, start 4 thread(s) * 25836 req/s * 84142 req/s * 28120 req/s *
* and do uncached sequential read * * * *
* by 4KB blocks * * * *
********************************************************************************
* Create/open 16 file(s) of total * 56006 req/s * 92137 req/s * 56992 req/s *
* size 2048.00 MB named * 55335 req/s * 92269 req/s * 57023 req/s *
* /tmp/tmp.tmp, start 4 thread(s) * 55731 req/s * 98722 req/s * 56593 req/s *
* and do uncached sequential read * * * *
* by 4KB blocks with constant * * * *
********************************************************************************
* Create/open 16 file(s) of total * 14104 req/s * 14164 req/s * 13914 req/s *
* size 2048.00 MB named * 14130 req/s * 14232 req/s * 13613 req/s *
* /tmp/tmp.tmp, start 4 thread(s) * 14183 req/s * 14080 req/s * 13374 req/s *
* and do uncached random read by * * * *
* 4KB blocks * * * *
********************************************************************************
* Create/open 16 file(s) of total * 23480 req/s * 23483 req/s * 20887 req/s *
* size 2048.00 MB named * 23070 req/s * 22432 req/s * 21127 req/s *
* /tmp/tmp.tmp, start 4 thread(s) * 24090 req/s * 23499 req/s * 23415 req/s *
* and do uncached random read by * * * *
* 4KB blocks with constant queue * * * *
* len 32 * * * *
********************************************************************************