Patchwork [08,of,10,V2] sparse-revlog: introduce native (C) implementation of slicechunktodensity

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Submitter Boris Feld
Date Nov. 15, 2018, 10:38 a.m.
Message ID <465af090febebc1c8ca8.1542278326@localhost.localdomain>
Download mbox | patch
Permalink /patch/36598/
State Accepted
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Boris Feld - Nov. 15, 2018, 10:38 a.m.
# HG changeset patch
# User Boris Feld <boris.feld@octobus.net>
# Date 1542276598 -3600
#      Thu Nov 15 11:09:58 2018 +0100
# Node ID 465af090febebc1c8ca8e402c279349ccd09e091
# Parent  f76fc4ed86fd2072c861f57a3c1c3e892648fc92
# EXP-Topic sparse-perf
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 465af090febe
sparse-revlog: introduce native (C) implementation of slicechunktodensity

This is a C implementation of `_slicechunktodensity` in the
`mercurial/revlogutils/deltas.py` file.

The algorithm involves a lot of integer manipulation and low-level access to
index data. Having a C implementation of it raises a large performance
improvement. See later changeset in this series for details.
Yuya Nishihara - Nov. 16, 2018, 1:40 p.m.
On Thu, 15 Nov 2018 11:38:46 +0100, Boris Feld wrote:
> # HG changeset patch
> # User Boris Feld <boris.feld@octobus.net>
> # Date 1542276598 -3600
> #      Thu Nov 15 11:09:58 2018 +0100
> # Node ID 465af090febebc1c8ca8e402c279349ccd09e091
> # Parent  f76fc4ed86fd2072c861f57a3c1c3e892648fc92
> # EXP-Topic sparse-perf
> # Available At https://bitbucket.org/octobus/mercurial-devel/
> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 465af090febe
> sparse-revlog: introduce native (C) implementation of slicechunktodensity
> 
> This is a C implementation of `_slicechunktodensity` in the
> `mercurial/revlogutils/deltas.py` file.
> 
> The algorithm involves a lot of integer manipulation and low-level access to
> index data. Having a C implementation of it raises a large performance
> improvement. See later changeset in this series for details.
> 
> diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
> --- a/mercurial/cext/revlog.c
> +++ b/mercurial/cext/revlog.c
> @@ -11,6 +11,7 @@
>  #include <assert.h>
>  #include <ctype.h>
>  #include <stddef.h>
> +#include <stdlib.h>
>  #include <string.h>
>  
>  #include "bitmanipulation.h"
> @@ -1050,6 +1051,210 @@ static PyObject *_trimchunk(indexObject 
>  	return chunk;
>  }
>  
> +struct Gap {
> +	Py_ssize_t size;
> +	Py_ssize_t idx;
> +};
> +
> +static int gap_compare(const void *left, const void *right)
> +{
> +	const struct Gap *_left = ((const struct Gap *)left);
> +	const struct Gap *_right = ((const struct Gap *)right);
> +	if (_left->size < _right->size) {
> +		return -1;
> +	} else if (_left->size > _right->size) {
> +		return 1;
> +	}
> +	return 0;
> +}
> +static int Py_ssize_t_compare(const void *left, const void *right)
> +{
> +	const Py_ssize_t _left = *(const Py_ssize_t *)left;
> +	const Py_ssize_t _right = *(const Py_ssize_t *)right;
> +	if (_left < _right) {
> +		return -1;
> +	} else if (_left > _right) {
> +		return 1;
> +	}
> +	return 0;

Nit: IIRC, names starting with _ is reserved for compilers. Better to not
use such variables.

> +static PyObject *index_slicechunktodensity(indexObject *self, PyObject *args)
> +{
> +	/* method arguments */
> +	PyObject *list_revs = NULL; /* revisions in the chain */
> +	double targetdensity = 0.5; /* min density to achieve */
> +	Py_ssize_t mingapsize = 0;  /* threshold to ignore gaps */

Nit: perhaps these default values, 0.5 and 0, wouldn't be used since function
arguments aren't optional.

This patch looks good to me.

As a follow up, can you somehow rewrite the doctests in deltas.py to run with
the C implementation? Since _testrevlog() isn't backed by the index object,
C functions aren't covered by the doctests. That's unfortunate.
Boris Feld - Nov. 19, 2018, 9:19 a.m.
On 16/11/2018 14:40, Yuya Nishihara wrote:
> On Thu, 15 Nov 2018 11:38:46 +0100, Boris Feld wrote:
>> # HG changeset patch
>> # User Boris Feld <boris.feld@octobus.net>
>> # Date 1542276598 -3600
>> #      Thu Nov 15 11:09:58 2018 +0100
>> # Node ID 465af090febebc1c8ca8e402c279349ccd09e091
>> # Parent  f76fc4ed86fd2072c861f57a3c1c3e892648fc92
>> # EXP-Topic sparse-perf
>> # Available At https://bitbucket.org/octobus/mercurial-devel/
>> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 465af090febe
>> sparse-revlog: introduce native (C) implementation of slicechunktodensity
>>
>> This is a C implementation of `_slicechunktodensity` in the
>> `mercurial/revlogutils/deltas.py` file.
>>
>> The algorithm involves a lot of integer manipulation and low-level access to
>> index data. Having a C implementation of it raises a large performance
>> improvement. See later changeset in this series for details.
>>
>> diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
>> --- a/mercurial/cext/revlog.c
>> +++ b/mercurial/cext/revlog.c
>> @@ -11,6 +11,7 @@
>>  #include <assert.h>
>>  #include <ctype.h>
>>  #include <stddef.h>
>> +#include <stdlib.h>
>>  #include <string.h>
>>  
>>  #include "bitmanipulation.h"
>> @@ -1050,6 +1051,210 @@ static PyObject *_trimchunk(indexObject 
>>  	return chunk;
>>  }
>>  
>> +struct Gap {
>> +	Py_ssize_t size;
>> +	Py_ssize_t idx;
>> +};
>> +
>> +static int gap_compare(const void *left, const void *right)
>> +{
>> +	const struct Gap *_left = ((const struct Gap *)left);
>> +	const struct Gap *_right = ((const struct Gap *)right);
>> +	if (_left->size < _right->size) {
>> +		return -1;
>> +	} else if (_left->size > _right->size) {
>> +		return 1;
>> +	}
>> +	return 0;
>> +}
>> +static int Py_ssize_t_compare(const void *left, const void *right)
>> +{
>> +	const Py_ssize_t _left = *(const Py_ssize_t *)left;
>> +	const Py_ssize_t _right = *(const Py_ssize_t *)right;
>> +	if (_left < _right) {
>> +		return -1;
>> +	} else if (_left > _right) {
>> +		return 1;
>> +	}
>> +	return 0;
> Nit: IIRC, names starting with _ is reserved for compilers. Better to not
> use such variables.
Oops, fixed.
>
>> +static PyObject *index_slicechunktodensity(indexObject *self, PyObject *args)
>> +{
>> +	/* method arguments */
>> +	PyObject *list_revs = NULL; /* revisions in the chain */
>> +	double targetdensity = 0.5; /* min density to achieve */
>> +	Py_ssize_t mingapsize = 0;  /* threshold to ignore gaps */
> Nit: perhaps these default values, 0.5 and 0, wouldn't be used since function
> arguments aren't optional.
I updated the code to set them both to 0. Should we leave them
non-initialized?
>
> This patch looks good to me.
>
> As a follow up, can you somehow rewrite the doctests in deltas.py to run with
> the C implementation? Since _testrevlog() isn't backed by the index object,
> C functions aren't covered by the doctests. That's unfortunate.
That won't be easy, the doctest use a mock python class to emulate the
revlog. On the other hand, the C code expects the real object.
> _______________________________________________
> Mercurial-devel mailing list
> Mercurial-devel@mercurial-scm.org
> https://www.mercurial-scm.org/mailman/listinfo/mercurial-devel
Yuya Nishihara - Nov. 19, 2018, 12:14 p.m.
On Mon, 19 Nov 2018 10:19:55 +0100, Boris FELD wrote:
> >> +	double targetdensity = 0.5; /* min density to achieve */
> >> +	Py_ssize_t mingapsize = 0;  /* threshold to ignore gaps */
> > Nit: perhaps these default values, 0.5 and 0, wouldn't be used since function
> > arguments aren't optional.
> I updated the code to set them both to 0. Should we leave them
> non-initialized?

I don't care much about that. 0 should be fine.

> > This patch looks good to me.
> >
> > As a follow up, can you somehow rewrite the doctests in deltas.py to run with
> > the C implementation? Since _testrevlog() isn't backed by the index object,
> > C functions aren't covered by the doctests. That's unfortunate.
> That won't be easy, the doctest use a mock python class to emulate the
> revlog. On the other hand, the C code expects the real object.

Can't they be extracted to unit tests? I don't think doctests fit well to
these somewhat complex functions.

Patch

diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c
+++ b/mercurial/cext/revlog.c
@@ -11,6 +11,7 @@ 
 #include <assert.h>
 #include <ctype.h>
 #include <stddef.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include "bitmanipulation.h"
@@ -1050,6 +1051,210 @@  static PyObject *_trimchunk(indexObject 
 	return chunk;
 }
 
+struct Gap {
+	Py_ssize_t size;
+	Py_ssize_t idx;
+};
+
+static int gap_compare(const void *left, const void *right)
+{
+	const struct Gap *_left = ((const struct Gap *)left);
+	const struct Gap *_right = ((const struct Gap *)right);
+	if (_left->size < _right->size) {
+		return -1;
+	} else if (_left->size > _right->size) {
+		return 1;
+	}
+	return 0;
+}
+static int Py_ssize_t_compare(const void *left, const void *right)
+{
+	const Py_ssize_t _left = *(const Py_ssize_t *)left;
+	const Py_ssize_t _right = *(const Py_ssize_t *)right;
+	if (_left < _right) {
+		return -1;
+	} else if (_left > _right) {
+		return 1;
+	}
+	return 0;
+}
+
+static PyObject *index_slicechunktodensity(indexObject *self, PyObject *args)
+{
+	/* method arguments */
+	PyObject *list_revs = NULL; /* revisions in the chain */
+	double targetdensity = 0.5; /* min density to achieve */
+	Py_ssize_t mingapsize = 0;  /* threshold to ignore gaps */
+
+	/* other core variables */
+	Py_ssize_t i;            /* used for various iteration */
+	PyObject *result = NULL; /* the final return of the function */
+
+	/* generic information about the delta chain being slice */
+	Py_ssize_t num_revs = 0;     /* size of the full delta chain */
+	Py_ssize_t *revs = NULL;     /* native array of revision in the chain */
+	Py_ssize_t chainpayload = 0; /* sum of all delta in the chain */
+	Py_ssize_t deltachainspan =
+	    0; /* distance from first byte to last byte */
+
+	/* variable used for slicing the delta chain */
+	Py_ssize_t readdata =
+	    0;              /* amount of data currently planned to be read */
+	double density = 0; /* ration of payload data compared to read ones */
+	struct Gap *gaps = NULL; /* array of notable gap in the chain */
+	Py_ssize_t num_gaps =
+	    0; /* total number of notable gap recorded so far */
+	Py_ssize_t *selected_indices = NULL; /* indices of gap skipped over */
+	Py_ssize_t num_selected = 0;         /* number of gaps skipped */
+	PyObject *chunk = NULL;              /* individual slice */
+	PyObject *allchunks = NULL;          /* all slices */
+
+	/* parsing argument */
+	if (!PyArg_ParseTuple(args, "O!dl", &PyList_Type, &list_revs,
+	                      &targetdensity, &mingapsize)) {
+		goto bail;
+	}
+
+	/* If the delta chain contains a single element, we do not need slicing
+	 */
+	num_revs = PyList_GET_SIZE(list_revs);
+	if (num_revs <= 1) {
+		result = PyTuple_Pack(1, list_revs);
+		goto done;
+	}
+
+	/* Turn the python list into a native integer array (for efficiency) */
+	revs = (Py_ssize_t *)calloc(num_revs, sizeof(Py_ssize_t));
+	if (revs == NULL) {
+		PyErr_NoMemory();
+		goto bail;
+	}
+	Py_ssize_t idxlen = index_length(self);
+	for (i = 0; i < num_revs; i++) {
+		Py_ssize_t revnum = PyInt_AsLong(PyList_GET_ITEM(list_revs, i));
+		if (revnum == -1 && PyErr_Occurred()) {
+			goto bail;
+		}
+		if (revnum < 0 || revnum >= idxlen) {
+			PyErr_SetString(PyExc_IndexError, "index out of range");
+			goto bail;
+		}
+		revs[i] = revnum;
+	}
+
+	/* Compute and check various property of the unsliced delta chain */
+	deltachainspan = index_segment_span(self, revs[0], revs[num_revs - 1]);
+
+	if (deltachainspan <= mingapsize) {
+		result = PyTuple_Pack(1, list_revs);
+		goto done;
+	}
+	chainpayload = 0;
+	for (i = 0; i < num_revs; i++) {
+		chainpayload += index_get_length(self, revs[i]);
+	}
+
+	readdata = deltachainspan;
+	density = 1.0;
+
+	if (0 < deltachainspan) {
+		density = (double)chainpayload / (double)deltachainspan;
+	};
+
+	if (density >= targetdensity) {
+		result = PyTuple_Pack(1, list_revs);
+		goto done;
+	}
+
+	/* if chain is too sparse, look for relevant gaps */
+	gaps = (struct Gap *)calloc(num_revs, sizeof(struct Gap));
+	if (gaps == NULL) {
+		PyErr_NoMemory();
+		goto bail;
+	}
+
+	Py_ssize_t previous_end = -1;
+	for (i = 0; i < num_revs; i++) {
+		Py_ssize_t revstart = index_get_start(self, revs[i]);
+		Py_ssize_t revsize = index_get_length(self, revs[i]);
+		if (revsize <= 0) {
+			continue;
+		}
+		if (previous_end >= 0) {
+			Py_ssize_t gapsize = revstart - previous_end;
+			if (gapsize > mingapsize) {
+				gaps[num_gaps].size = gapsize;
+				gaps[num_gaps].idx = i;
+				num_gaps += 1;
+			}
+		}
+		previous_end = revstart + revsize;
+	}
+	if (num_gaps == 0) {
+		result = PyTuple_Pack(1, list_revs);
+		goto done;
+	}
+	qsort(gaps, num_gaps, sizeof(struct Gap), &gap_compare);
+
+	/* Slice the largest gap first, they improve the density the most */
+	selected_indices =
+	    (Py_ssize_t *)malloc((num_gaps + 1) * sizeof(Py_ssize_t));
+	if (selected_indices == NULL) {
+		PyErr_NoMemory();
+		goto bail;
+	}
+
+	for (i = num_gaps - 1; i >= 0; i--) {
+		selected_indices[num_selected] = gaps[i].idx;
+		readdata -= gaps[i].size;
+		num_selected += 1;
+		if (readdata <= 0) {
+			density = 1.0;
+		} else {
+			density = (double)chainpayload / (double)readdata;
+		}
+		if (density >= targetdensity) {
+			break;
+		}
+	}
+	qsort(selected_indices, num_selected, sizeof(Py_ssize_t),
+	      &Py_ssize_t_compare);
+
+	/* create the resulting slice */
+	allchunks = PyList_New(0);
+	if (allchunks == NULL) {
+		goto bail;
+	}
+	Py_ssize_t previdx = 0;
+	selected_indices[num_selected] = num_revs;
+	for (i = 0; i <= num_selected; i++) {
+		Py_ssize_t idx = selected_indices[i];
+		chunk = _trimchunk(self, list_revs, previdx, idx);
+		if (chunk == NULL) {
+			goto bail;
+		}
+		if (PyList_GET_SIZE(chunk) > 0) {
+			if (PyList_Append(allchunks, chunk) == -1) {
+				goto bail;
+			}
+		}
+		Py_DECREF(chunk);
+		chunk = NULL;
+		previdx = idx;
+	}
+	result = allchunks;
+	goto done;
+
+bail:
+	Py_XDECREF(allchunks);
+	Py_XDECREF(chunk);
+done:
+	free(revs);
+	free(gaps);
+	free(selected_indices);
+	return result;
+}
+
 static inline int nt_level(const char *node, Py_ssize_t level)
 {
 	int v = node[level >> 1];
@@ -2282,6 +2487,8 @@  static PyMethodDef index_methods[] = {
      "get filtered head revisions"}, /* Can always do filtering */
     {"deltachain", (PyCFunction)index_deltachain, METH_VARARGS,
      "determine revisions with deltas to reconstruct fulltext"},
+    {"slicechunktodensity", (PyCFunction)index_slicechunktodensity,
+     METH_VARARGS, "determine revisions with deltas to reconstruct fulltext"},
     {"append", (PyCFunction)index_append, METH_O, "append an index entry"},
     {"partialmatch", (PyCFunction)index_partialmatch, METH_VARARGS,
      "match a potentially ambiguous node ID"},