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bug#23113: parallel gzip processes trash hard disks, need larger buffers

From: Chevreux, Bastien
Subject: bug#23113: parallel gzip processes trash hard disks, need larger buffers
Date: Fri, 25 Mar 2016 16:57:12 +0000

Hi there,

I am using gzip 1.6 to compress large files >10 GiB in parallel (Kubuntu 14.04, 
12 cores). The underlying disk system (RAID 10) is able to deliver read speeds 
>1 GB/s (measured with flushed file caches, iostat -mx 1 100).

Here are some numbers when running gzip in parallel:
1 gzip process: the CPU is the bottleneck in compressing things and utilisation 
is 100%.
2 gzips in parallel: the disk throughput drops to a meagre 70MB/s and the CPU 
utilisation per process is at ~60%.
6 gzips in parallel: the disk throughput fluctuates between 50 and 60 MB/s and 
the CPU utilisation per process is at ~18-20%.

Running 6 gzips in parallel on the same data residing on a SSD: 100% CPU 
utilisation per process

Googling a bit I found this thread on SuperUser where someone saw the same 
behaviour already with a single disk doing normally 125 MB/s and running 4 
gzips drops it to 25 MB/s:

The posts there propose a workaround like this:
  buffer -s 100000 -m 10000000 -p 100 < bigfile.dat | gzip > bigfile.dat.gz

And indeed, using "buffer" resolves trashing problems when working on a disk 
system. However, using "buffer" is pretty arcane (it isn't even installed per 
default on most Unix/Linux installations) and pretty counterintuitive.

Would it be possible to have bigger buffers by default (1 MB? 10 MB?) or have 
an automatism in gzip like "if file to compress >10 MB and free RAM >500MB, 
setup the file buffer to use 1 (10?) MB" ?

Alternatively, a command line option to manually set the buffer size?


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