Use pooled gzip.{Writer,Reader} in gzip{Compressor,Decompressor}

[steeve]

This change saves a lot of memory by reusing the underlying
gzip.{Writer,Reader}, which allocates up to 1.4mb at every instanciation
according to [1]. This was fixed by adding a Reset method by to the
object at [2].

The amount of memory (and GC time) saved is pretty high, as reported by
pprof for the benchmarks below:

      flat  flat%   sum%        cum   cum%
   28.33GB 85.70% 85.70%    32.74GB 99.05%  compress/flate.NewWriter

      flat  flat%   sum%        cum   cum%
   19.39MB 16.74% 16.74%    22.07MB 19.05%  compress/flate.NewWriter

And the benchmarks:

benchmark                           old ns/op     new ns/op     delta
BenchmarkGZIPCompressor1B-4         215170        22291         -89.64%
BenchmarkGZIPCompressor1KiB-4       225971        27213         -87.96%
BenchmarkGZIPCompressor8KiB-4       246696        54785         -77.79%
BenchmarkGZIPCompressor64KiB-4      444851        286924        -35.50%
BenchmarkGZIPCompressor512KiB-4     2279043       2115863       -7.16%
BenchmarkGZIPCompressor1MiB-4       4412989       4258635       -3.50%

benchmark                           old allocs     new allocs     delta
BenchmarkGZIPCompressor1B-4         17             0              -100.00%
BenchmarkGZIPCompressor1KiB-4       17             0              -100.00%
BenchmarkGZIPCompressor8KiB-4       17             0              -100.00%
BenchmarkGZIPCompressor64KiB-4      17             0              -100.00%
BenchmarkGZIPCompressor512KiB-4     17             0              -100.00%
BenchmarkGZIPCompressor1MiB-4       17             0              -100.00%

benchmark                           old bytes     new bytes     delta
BenchmarkGZIPCompressor1B-4         813872        8             -100.00%
BenchmarkGZIPCompressor1KiB-4       813872        16            -100.00%
BenchmarkGZIPCompressor8KiB-4       813875        27            -100.00%
BenchmarkGZIPCompressor64KiB-4      813918        190           -99.98%
BenchmarkGZIPCompressor512KiB-4     814928        1871          -99.77%
BenchmarkGZIPCompressor1MiB-4       820889        9735          -98.81%

[1] golang/go#6138
[2] golang/go@db12f9d

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[steeve]

I signed it!

[googlebot]

CLAs look good, thanks!

[steeve]

On a side note, this fixed the GC issues we had in production. Processes went from pretty much doing GC all the time back to a normal pattern.

Scheduling was also significantly improved as traces showed up to 800 runable goroutines all the time down to ~20 during GC, and 0 most of the time.

[dfawley]

Thanks for the contribution.

LGTM

[dfawley]

Whoops, I just noticed the test failure. TestCompress is instantiating its own compressor, so it should be changed to use NewGZIPCompressor() instead.

Tests fail.

[steeve]

Seems it was not pushed, fixed

[dfawley]

Thanks for the fix!