view mercurial/diffhelpers.c @ 30212:260af19891f2

changegroup: increase write buffer size to 128k

By default, Python defers to the operating system for choosing the
default buffer size on opened files. On my Linux machine, the default
is 4k, which is really small for 2016.

This patch bumps the write buffer size when writing
changegroups/bundles to 128k. This matches the 128k read buffer
we already use on revlogs.

It's worth noting that this only impacts when writing to an explicit
file (such as during `hg bundle`). Buffers when writing to bundle
files via the repo vfs or to a temporary file are not impacted.

When producing a none-v2 bundle file of the mozilla-unified repository,
this change caused the number of write() system calls to drop from
952,449 to 29,788. After this change, the most frequent system
calls are fstat(), read(), lseek(), and open(). There were
2,523,672 system calls after this patch (so a net decrease of
~950k is statistically significant).

This change shows no performance change on my system. But I have a
high-end system with a fast SSD. It is quite possible this change
will have a significant impact on network file systems, where
extra network round trips due to excessive I/O system calls could
introduce significant latency.

/*
 * diffhelpers.c - helper routines for mpatch
 *
 * Copyright 2007 Chris Mason <[e-mail]>
 *
 * This software may be used and distributed according to the terms
 * of the GNU General Public License v2, incorporated herein by reference.
 */

#include <Python.h>
#include <stdlib.h>
#include <string.h>

#include "util.h"

static char diffhelpers_doc[] = "Efficient diff parsing";
static PyObject *diffhelpers_Error;


/* fixup the last lines of a and b when the patch has no newline at eof */
static void _fix_newline(PyObject *hunk, PyObject *a, PyObject *b)
{
	Py_ssize_t hunksz = PyList_Size(hunk);
	PyObject *s = PyList_GET_ITEM(hunk, hunksz-1);
	char *l = PyBytes_AsString(s);
	Py_ssize_t alen = PyList_Size(a);
	Py_ssize_t blen = PyList_Size(b);
	char c = l[0];
	PyObject *hline;
	Py_ssize_t sz = PyBytes_GET_SIZE(s);

	if (sz > 1 && l[sz-2] == '\r')
		/* tolerate CRLF in last line */
		sz -= 1;

	hline = PyBytes_FromStringAndSize(l, sz-1);
	if (!hline) {
		return;
	}

	if (c == ' ' || c == '+') {
		PyObject *rline = PyBytes_FromStringAndSize(l + 1, sz - 2);
		PyList_SetItem(b, blen-1, rline);
	}
	if (c == ' ' || c == '-') {
		Py_INCREF(hline);
		PyList_SetItem(a, alen-1, hline);
	}
	PyList_SetItem(hunk, hunksz-1, hline);
}

/* python callable form of _fix_newline */
static PyObject *
fix_newline(PyObject *self, PyObject *args)
{
	PyObject *hunk, *a, *b;
	if (!PyArg_ParseTuple(args, "OOO", &hunk, &a, &b))
		return NULL;
	_fix_newline(hunk, a, b);
	return Py_BuildValue("l", 0);
}

#if (PY_VERSION_HEX < 0x02050000)
static const char *addlines_format = "OOiiOO";
#else
static const char *addlines_format = "OOnnOO";
#endif

/*
 * read lines from fp into the hunk.  The hunk is parsed into two arrays
 * a and b.  a gets the old state of the text, b gets the new state
 * The control char from the hunk is saved when inserting into a, but not b
 * (for performance while deleting files)
 */
static PyObject *
addlines(PyObject *self, PyObject *args)
{

	PyObject *fp, *hunk, *a, *b, *x;
	Py_ssize_t i;
	Py_ssize_t lena, lenb;
	Py_ssize_t num;
	Py_ssize_t todoa, todob;
	char *s, c;
	PyObject *l;
	if (!PyArg_ParseTuple(args, addlines_format,
			      &fp, &hunk, &lena, &lenb, &a, &b))
		return NULL;

	while (1) {
		todoa = lena - PyList_Size(a);
		todob = lenb - PyList_Size(b);
		num = todoa > todob ? todoa : todob;
		if (num == 0)
		    break;
		for (i = 0; i < num; i++) {
			x = PyFile_GetLine(fp, 0);
			s = PyBytes_AsString(x);
			c = *s;
			if (strcmp(s, "\\ No newline at end of file\n") == 0) {
				_fix_newline(hunk, a, b);
				continue;
			}
			if (c == '\n') {
				/* Some patches may be missing the control char
				 * on empty lines. Supply a leading space. */
				Py_DECREF(x);
				x = PyBytes_FromString(" \n");
			}
			PyList_Append(hunk, x);
			if (c == '+') {
				l = PyBytes_FromString(s + 1);
				PyList_Append(b, l);
				Py_DECREF(l);
			} else if (c == '-') {
				PyList_Append(a, x);
			} else {
				l = PyBytes_FromString(s + 1);
				PyList_Append(b, l);
				Py_DECREF(l);
				PyList_Append(a, x);
			}
			Py_DECREF(x);
		}
	}
	return Py_BuildValue("l", 0);
}

/*
 * compare the lines in a with the lines in b.  a is assumed to have
 * a control char at the start of each line, this char is ignored in the
 * compare
 */
static PyObject *
testhunk(PyObject *self, PyObject *args)
{

	PyObject *a, *b;
	long bstart;
	Py_ssize_t alen, blen;
	Py_ssize_t i;
	char *sa, *sb;

	if (!PyArg_ParseTuple(args, "OOl", &a, &b, &bstart))
		return NULL;
	alen = PyList_Size(a);
	blen = PyList_Size(b);
	if (alen > blen - bstart || bstart < 0) {
		return Py_BuildValue("l", -1);
	}
	for (i = 0; i < alen; i++) {
		sa = PyBytes_AsString(PyList_GET_ITEM(a, i));
		sb = PyBytes_AsString(PyList_GET_ITEM(b, i + bstart));
		if (strcmp(sa + 1, sb) != 0)
			return Py_BuildValue("l", -1);
	}
	return Py_BuildValue("l", 0);
}

static PyMethodDef methods[] = {
	{"addlines", addlines, METH_VARARGS, "add lines to a hunk\n"},
	{"fix_newline", fix_newline, METH_VARARGS, "fixup newline counters\n"},
	{"testhunk", testhunk, METH_VARARGS, "test lines in a hunk\n"},
	{NULL, NULL}
};

#ifdef IS_PY3K
static struct PyModuleDef diffhelpers_module = {
	PyModuleDef_HEAD_INIT,
	"diffhelpers",
	diffhelpers_doc,
	-1,
	methods
};

PyMODINIT_FUNC PyInit_diffhelpers(void)
{
	PyObject *m;

	m = PyModule_Create(&diffhelpers_module);
	if (m == NULL)
		return NULL;

	diffhelpers_Error = PyErr_NewException("diffhelpers.diffhelpersError",
											NULL, NULL);
	Py_INCREF(diffhelpers_Error);
	PyModule_AddObject(m, "diffhelpersError", diffhelpers_Error);

	return m;
}
#else
PyMODINIT_FUNC
initdiffhelpers(void)
{
	Py_InitModule3("diffhelpers", methods, diffhelpers_doc);
	diffhelpers_Error = PyErr_NewException("diffhelpers.diffhelpersError",
	                                        NULL, NULL);
}
#endif