Re: [PATCH 1/3] sha1_name: Create perf test for find_unique_abbrev()

[Junio C Hamano]

Derrick Stolee  writes:

>> But I do not think we want this "clever" optimization that involves
>> 'n' in the first place.
 +  while (count++ < 10) {
>>> +   for (i = 0; i < n; i++)
>>> +   ((unsigned int*)oid.hash)[i] = hash_base;
>>
>> Does it make sense to assume that uint is always 4-byte (so this
>> code won't work if it is 8-byte on your platform) and doing this is
>> faster than using platform-optimized memcpy()?
>
> I'm not sure what you mean by using memcpy to improve this, because
> it would require calling memcpy in the inner loop, such as
>
>   for (i = 0; i < n; i++)
>   memcpy(oid.hash + i * sizeof(unsigned), _base,
>  sizeof(unsigned));

Sorry, I left it without saying as I thought it was obvious, but
what I meant was to use a whole "struct oid", not just a single
unsigned (repeated), as the hash that is tested.  If you have an
array of object names you use in the test, then

for (count = 0; count < limit; count++) {
hashcpy(, [count]);

... do the probing ...
}

> First, this doesn't just measure the time it takes to determine non-
> existence,

Sorry, my phrasing was indeed misleading.  I know the time we spend
to see if we have or do not have the object is the largest cycle
spender in these codepaths (and even if it were, I do not think that
is what you are trying to optimize in these patches anyway).  

But if I recall correctly, the way we come up with the unique
abbreviation for an object that exists and an object that does not
exist are different?  And because most of the time (think: "git log
-p" output) we would be finding abbreviation for objects that we do
have, benchmarking and tweaking the code that comes up with an
object that does not exist is not optimizing for the right case.

Back when I wrote that initial response, I didn't check how
different the code was between the two cases, but now I did.  It
seems that in both cases we start from the shortest-allowed length
and then extend the same way, and the only difference between these
two cases is that we return immediately when our candidate name is
long enough not to match any existing object when the full name
refers to an object we do not have, while we return only when
disambiguity is resolved.  I _think_ these amount to the same
computation (i.e. when an object with the full name we have exists,
the amount of computation we need to come up with its unique
abbreviation is the same as the computation we need to find the
unique abbreviation for the same name in another repository that has
identical set of objects, minus that single object), so from that
point of view, throwing random hashes, most of which would not name
any existing object, and measuring how much time it takes to run
get_short_oid() to compute the minimum length of the unique prefix
should be sufficient.

Thanks.

Re: [PATCH 1/3] sha1_name: Create perf test for find_unique_abbrev()

[Derrick Stolee]

On 9/17/2017 5:51 PM, Junio C Hamano wrote:

Derrick Stolee  writes:


+int cmd_main(int ac, const char **av)
+{
+   setup_git_directory();


As far as I recall, we do not (yet) allow declaration after
statement in our codebase.  Move this down to make it after all
decls.


Will fix.


+
+   unsigned int hash_delt = 0x13579BDF;
+   unsigned int hash_base = 0x01020304;
+   struct object_id oid;
+
+   int i, count = 0;
+   int n = sizeof(struct object_id) / sizeof(int);


It probably is technically OK to assume sizeof(int) always equals to
sizeof(unsigned), but because you use 'n' _only_ to work with uint
and never with int, it would make more sense to match.


Will fix. I also notice that "n" should be const.


But I do not think we want this "clever" optimization that involves
'n' in the first place.

>>> +  while (count++ < 10) {

+   for (i = 0; i < n; i++)
+   ((unsigned int*)oid.hash)[i] = hash_base;


Does it make sense to assume that uint is always 4-byte (so this
code won't work if it is 8-byte on your platform) and doing this is
faster than using platform-optimized memcpy()?


I'm not sure what you mean by using memcpy to improve this, because
it would require calling memcpy in the inner loop, such as

for (i = 0; i < n; i++)
memcpy(oid.hash + i * sizeof(unsigned), _base,
   sizeof(unsigned));

I'm probably misunderstanding your intended use of memcpy().


+   find_unique_abbrev(oid.hash, MINIMUM_ABBREV);
+
+   hash_base += hash_delt;
+   }


I also wonder if this is measuring the right thing.  I am guessing
that by making many queries for a unique abbreviation of "random"
(well, it is deterministic, but my point is these queries are not
based on the names of objects that exist in the repository) hashes,
this test measures how much time it takes for us to decide that such
a random hash does not exist.  In the real life use, we make the
opposite query far more frequently: we have an object that we _know_
exists in the repository and we try to find a sufficient length to
disambiguate it from others, and I suspect that these two use
different logic.  Don't you need to be measuring the time it takes
to compute the shortest abbreviation of an object that exists
instead?


First, this doesn't just measure the time it takes to determine non-
existence, because it finds the len required to disambiguate an
"incoming" hash from all known hashes. When testing, I put in a
simple printf to report the result abbreviation so I could see how
often it needed to be extended. In this sense, the test exposes the
while loop that is removed by PATCH 2/3.

Second, your criticism about extant hashes is valid, and one I
struggled to reconcile. I see two issues with testing known hashes:

1. By determining the hash exists, we have inspected the file that
   contains it (either a .idx or the loose-object directory). This
   has side-effects that warm up the file cache so the looped method
   is artificially faster to find matching objects. The effect is
   particularly significant on a repo with exactly one packfile.

2. If we iterate over the entire set of objects, this test takes
   O(N*t(N)) time, where t(N) is the average time to compute a
   minimum abbreviation. For large repos, this can take several
   minutes. Instead, even with the constant 100,000 test hashes, we
   have an O(t(N)) test. We could avoid the asymptotic growth by
   limiting the number of existing hashes we use, but how do we
   find a sufficiently uniform sample from them?

By looking at some other perf tests, I see that we can add a pre-
requisite action. I will investigate making another helper that
uniformly selects a set of hashes from the repo and writes them
to stdout in a random order. p0008-abbrev.sh will run the helper as
a prerequisite, saving the data to a file. test-abbrev will read
the hashes from stdin to test find_unique_abbrev. This should avoid
the side-effects I mentioned (test-abbrev will not warm up indexes)
while also testing abbreviation lengths for existing objects.

I'll get started on these changes and send a new patch with new perf
data in a couple days. Please let me know if there is a better way
to measure performance for this change.

Thanks,
-Stolee

Re: [PATCH 1/3] sha1_name: Create perf test for find_unique_abbrev()

[Junio C Hamano]

Derrick Stolee  writes:

> +int cmd_main(int ac, const char **av)
> +{
> + setup_git_directory();

As far as I recall, we do not (yet) allow declaration after
statement in our codebase.  Move this down to make it after all
decls.

> +
> + unsigned int hash_delt = 0x13579BDF;
> + unsigned int hash_base = 0x01020304;
> + struct object_id oid;
> +
> + int i, count = 0;
> + int n = sizeof(struct object_id) / sizeof(int);

It probably is technically OK to assume sizeof(int) always equals to
sizeof(unsigned), but because you use 'n' _only_ to work with uint
and never with int, it would make more sense to match.  

But I do not think we want this "clever" optimization that involves
'n' in the first place.

> + while (count++ < 10) {
> + for (i = 0; i < n; i++)
> + ((unsigned int*)oid.hash)[i] = hash_base;

Does it make sense to assume that uint is always 4-byte (so this
code won't work if it is 8-byte on your platform) and doing this is
faster than using platform-optimized memcpy()?

> + find_unique_abbrev(oid.hash, MINIMUM_ABBREV);
> +
> + hash_base += hash_delt;
> + }

I also wonder if this is measuring the right thing.  I am guessing
that by making many queries for a unique abbreviation of "random"
(well, it is deterministic, but my point is these queries are not
based on the names of objects that exist in the repository) hashes,
this test measures how much time it takes for us to decide that such
a random hash does not exist.  In the real life use, we make the
opposite query far more frequently: we have an object that we _know_
exists in the repository and we try to find a sufficient length to
disambiguate it from others, and I suspect that these two use
different logic.  Don't you need to be measuring the time it takes
to compute the shortest abbreviation of an object that exists
instead?

> + exit(0);
> +}
> diff --git a/t/perf/p0008-abbrev.sh b/t/perf/p0008-abbrev.sh
> new file mode 100755
> index 0..7c3fad807
> --- /dev/null
> +++ b/t/perf/p0008-abbrev.sh
> @@ -0,0 +1,12 @@
> +#!/bin/sh
> +
> +test_description='Test object disambiguation through abbreviations'
> +. ./perf-lib.sh
> +
> +test_perf_large_repo
> +
> +test_perf 'find_unique_abbrev()' '
> + test-abbrev
> +'
> +
> +test_done

[PATCH 1/3] sha1_name: Create perf test for find_unique_abbrev()

[Derrick Stolee]

Create helper program test-abbrev to compute the minimum length of a
disambiguating short-sha for 100,000 object ids. The ids are created
by iterating an unsigned int hash_base by a constant hash_delta and
copying hash_base five times across the sha1. Iterating by hash_delta
does not create a duplicate value for over 10,000,000 iterations.

test-abberv demonstrates the performance improvements that will be
shown by later improvements to the find_unique_abberv(). The value of
100,000 is large enough to show the significance of the later
improvements while only taking a few seconds on large repos.

Signed-off-by: Derrick Stolee 
---
 Makefile   |  1 +
 t/helper/.gitignore|  1 +
 t/helper/test-abbrev.c | 23 +++
 t/perf/p0008-abbrev.sh | 12 
 4 files changed, 37 insertions(+)
 create mode 100644 t/helper/test-abbrev.c
 create mode 100755 t/perf/p0008-abbrev.sh

diff --git a/Makefile b/Makefile
index f2bb7f2f6..081ca05e8 100644
--- a/Makefile
+++ b/Makefile
@@ -633,6 +633,7 @@ X =
 
 PROGRAMS += $(patsubst %.o,git-%$X,$(PROGRAM_OBJS))
 
+TEST_PROGRAMS_NEED_X += test-abbrev
 TEST_PROGRAMS_NEED_X += test-chmtime
 TEST_PROGRAMS_NEED_X += test-ctype
 TEST_PROGRAMS_NEED_X += test-config
diff --git a/t/helper/.gitignore b/t/helper/.gitignore
index 721650256..80ce7d836 100644
--- a/t/helper/.gitignore
+++ b/t/helper/.gitignore
@@ -1,3 +1,4 @@
+/test-abbrev
 /test-chmtime
 /test-ctype
 /test-config
diff --git a/t/helper/test-abbrev.c b/t/helper/test-abbrev.c
new file mode 100644
index 0..cb3551df9
--- /dev/null
+++ b/t/helper/test-abbrev.c
@@ -0,0 +1,23 @@
+#include "cache.h"
+
+int cmd_main(int ac, const char **av)
+{
+   setup_git_directory();
+
+   unsigned int hash_delt = 0x13579BDF;
+   unsigned int hash_base = 0x01020304;
+   struct object_id oid;
+
+   int i, count = 0;
+   int n = sizeof(struct object_id) / sizeof(int);
+   while (count++ < 10) {
+   for (i = 0; i < n; i++)
+   ((unsigned int*)oid.hash)[i] = hash_base;
+
+   find_unique_abbrev(oid.hash, MINIMUM_ABBREV);
+
+   hash_base += hash_delt;
+   }
+
+   exit(0);
+}
diff --git a/t/perf/p0008-abbrev.sh b/t/perf/p0008-abbrev.sh
new file mode 100755
index 0..7c3fad807
--- /dev/null
+++ b/t/perf/p0008-abbrev.sh
@@ -0,0 +1,12 @@
+#!/bin/sh
+
+test_description='Test object disambiguation through abbreviations'
+. ./perf-lib.sh
+
+test_perf_large_repo
+
+test_perf 'find_unique_abbrev()' '
+   test-abbrev
+'
+
+test_done
-- 
2.14.1.538.g56ec8fc98.dirty