Subject: Re: test-tool: bloom: generate_filter for multiple string?

From: Đoàn Trần Công Danh <[email-masked]>
Date: Tue, 5 Jan 2021 20:34:56 +0700

On 2020-12-31 06:31:52-0500, Derrick Stolee <[email-masked]> wrote:
> On 12/30/2020 10:54 PM, Đoàn Trần Công Danh wrote:
> > I'm reading the code for Bloom Filter to see if arXiv:2012.00472
> > could be an improvement.
> > 
> > I'm not sure if I'm missing something or it's our test-bloom generate_filter
> > doesn't really support testing for multiple inputs.
> 
> It doesn't support creating a _realistic_ filter of the given size. The tests
> at this level are more about keeping the filters consistent so we can trust
> that we are not accidentally changing the hashing algorithm and its file format.

(Sorry for this late reply)

Yes, make sense.

> The test works if we provide multiple inputs, the problem is that the resulting
> filter has a higher density of bits than we expect, since we didn't size it
> according to the number of inputs.

OK.

> > If I understand correctly, we should either:
> > * allocate more entry for inputs; or
> > * abort if argc != 3
> 
> Either approach would be fine. I wonder what your goal is for testing the
> multiple inputs. Are you expecting a larger filter to demonstrate some
> behavior that is not tested by a small filter?

I was expecting a larger filter to trying with some ideas that I had
in mind before implement it in real code.
However, after toying with my idea, I think it's more of a solution
looking for problem.
(The original post was posted before I realised that).

> > diff --git a/t/helper/test-bloom.c b/t/helper/test-bloom.c
> > index 46e97b04eb..1026facc59 100644
> > --- a/t/helper/test-bloom.c
> > +++ b/t/helper/test-bloom.c
> > @@ -68,12 +68,14 @@ int cmd__bloom(int argc, const char **argv)
> >  	if (!strcmp(argv[1], "generate_filter")) {
> >  		struct bloom_filter filter;
> >  		int i = 2;
> > -		filter.len =  (settings.bits_per_entry + BITS_PER_WORD - 1) / BITS_PER_WORD;
> > -		filter.data = xcalloc(filter.len, sizeof(unsigned char));
> >  
> >  		if (argc - 1 < i)
> >  			usage(bloom_usage);
> >  
> > +		filter.len = (settings.bits_per_entry * (argc - 2) + BITS_PER_WORD - 1)
> > +			/ BITS_PER_WORD;
> > +		filter.data = xcalloc(filter.len, sizeof(unsigned char));
> > +
> 
> Whitespace issues aside, this is the right approach to make the filter grow with
> the input.
> 
> >  		while (argv[i]) {
> >  			add_string_to_filter(argv[i], &filter);
> >  			i++;
> > diff --git a/t/t0095-bloom.sh b/t/t0095-bloom.sh
> > index 7e4ab1795f..6d83927c86 100755
> > --- a/t/t0095-bloom.sh
> > +++ b/t/t0095-bloom.sh
> > @@ -67,6 +67,17 @@ test_expect_success 'compute bloom key for test string 2' '
> >  	test_cmp expect actual
> >  '
> >  
> > +test_expect_success 'compute bloom key for multiple string' '
> > +	cat >expect <<-\EOF &&
> > +	Hashes:0xb270de9b|0x1bb6f26e|0x84fd0641|0xee431a14|0x57892de7|0xc0cf41ba|0x2a15558d|
> > +	Hashes:0x20ab385b|0xf5237fe2|0xc99bc769|0x9e140ef0|0x728c5677|0x47049dfe|0x1b7ce585|
> 
> Multiple hash outputs work already, helping us build confidence that our
> hash algorithm hasn't changed.
> 
> > +	Filter_Length:3
> > +	Filter_Data:45|ba|ac|
> 
> And this is the part of the output that you are changing.
> 
> > +	EOF
> > +	test-tool bloom generate_filter "Hello world!" file.txt >actual &&
> > +	test_cmp expect actual
> > +'
> > +
> 
> So, your suggested change has merit. I just wonder what value it provides on top
> of the current implementation? If your goal is to create a way to inspect a full-sized
> filter, then that would be interesting to explore.

Yes, originally, I was trying to creat some way to inspect a full-size
filter, i.e. a bloom filter for some set of object_id during fetch
negotiation.
But, I realise it's not a good idea, now.

-- 
Danh