Re: [PATCH 1/2] add `#kgio_writev` and `#kgio_trywritev`

[Eric Wong <normalperson@yhbt.net>]

Sokolov Yura 'funny-falcon <funny.falcon@gmail.com> wrote:
> Add methods for using writev(2) syscall for sending array of string in
> a single syscall. This is more efficient than concatenating strings on
> Ruby side.
> `#kgio_trywritev` returns array of strings which are not sent to the
> socket.

Thanks.  Comments inline.

> Since both methods dups array and strings in it, `#kgio_writev` semantic
> a bit different from `#kgio_write`: it does not react on changes to
> array/strings that made in other thread. But I think, this way is more
> correct.

Perhaps we should use rb_str_locktmp()?

> --- a/ext/kgio/read_write.c
> +++ b/ext/kgio/read_write.c
> @@ -1,6 +1,9 @@
>  #include "kgio.h"
>  #include "my_fileno.h"
>  #include "nonblock.h"
> +#ifdef HAVE_WRITEV
> +#  include "sys/uio.h"
> +#endif

Use angle braces for system headers: <sys/uio.h>

>  static VALUE sym_wait_readable, sym_wait_writable;
>  static VALUE eErrno_EPIPE, eErrno_ECONNRESET;
>  static ID id_set_backtrace;
> @@ -403,6 +406,158 @@ static VALUE kgio_trywrite(VALUE io, VALUE str)
>  	return my_write(io, str, 0);
>  }
>  
> +struct io_args_v {
> +    VALUE io;
> +    VALUE buf;
> +    VALUE vec_buf;
> +    struct iovec *vec;
> +    int rest_len;
> +    int total_len;

size_t is better for lengths.  Or long if you want RSTRING compat
but definitely not int.

Also, use hard tabs for all indentations.

> +#ifdef HAVE_WRITEV
> +static void prepare_writev(struct io_args_v *a, VALUE io, VALUE ary)
> +{
> +	long i;
> +	a->io = io;
> +	a->fd = my_fileno(io);
> +	a->buf = rb_ary_dup(ary);
> +	a->vec_buf = rb_str_tmp_new(sizeof(struct iovec) * RARRAY_LEN(a->buf));
> +	a->vec = (struct iovec*)RSTRING_PTR(a->vec_buf);
> +	a->total_len = 0;
> +	for(i=0; i < RARRAY_LEN(a->buf); i++) {

Formatting:

	for (i = 0; ...

> +		VALUE str = RARRAY_PTR(a->buf)[i];
> +		if (TYPE(str) != T_STRING) {
> +			str = rb_obj_as_string(str);
> +		} else {
> +			str = rb_str_dup_frozen(str);
> +		}
> +		RARRAY_PTR(a->buf)[i] = str;
> +		a->vec[i].iov_base = RSTRING_PTR(str);
> +		a->vec[i].iov_len = RSTRING_LEN(str);
> +		a->total_len += RSTRING_LEN(str);

Avoid repeatedly calling RARRAY_*/RSTRING_* macros on the same object.
It adds noise and makes the code harder to read (especially with CAPS).
There's also a small size reduction because these macros branch in MRI
1.9+

> +	}
> +	a->rest_len = a->total_len;
> +}
> +
> +static int writev_check(struct io_args_v *a, long n, const char *msg, int io_wait)
> +{
> +	if (a->rest_len == n) {
> +done:
> +		a->buf = Qnil;
> +	} else if (n == -1) {
> +		if (errno == EINTR) {
> +			a->fd = my_fileno(a->io);
> +			return -1;
> +		}
> +		if (errno == EAGAIN) {
> +			if (io_wait) {
> +				(void)kgio_call_wait_writable(a->io);
> +				return -1;
> +			} else if (a->total_len == a->rest_len) {
> +				a->buf = sym_wait_writable;
> +			}
> +			return 0;
> +		}
> +		wr_sys_fail(msg);
> +	} else {
> +		assert(n >= 0 && n < a->rest_len && "writev syscall broken?");
> +		a->rest_len -= n;
> +		while (n > 0) {
> +			VALUE str = RARRAY_PTR(a->buf)[0];
> +			if (RSTRING_LEN(str) > n) {
> +				str = rb_str_subseq(str, n, RSTRING_LEN(str) - n);
> +				RARRAY_PTR(a->buf)[0] = str;
> +				a->vec->iov_base = RSTRING_PTR(str);
> +				a->vec->iov_len = RSTRING_LEN(str);
> +				n = 0;
> +			} else {
> +				n -= RSTRING_LEN(str);
> +				rb_ary_shift(a->buf);
> +				a->vec++;

Probably better to store the array offset instead of shifting
Array#shift is O(n).

> +			}
> +		}
> +		return -1;
> +	}
> +	return 0;
> +}
> +
> +static VALUE my_writev(VALUE io, VALUE str, int io_wait)
> +{
> +	struct io_args_v a;
> +	long n, iov_cnt, iov_max;
> +
> +	prepare_writev(&a, io, str);
> +	set_nonblocking(a.fd);
> +	iov_max = sysconf(_SC_IOV_MAX);
> +retry:
> +	iov_cnt = RARRAY_LEN(a.buf);
> +	if (iov_cnt > iov_max) iov_cnt = iov_max;
> +	n = (long)writev(a.fd, a.vec, iov_cnt);
> +	if (writev_check(&a, n, "writev", io_wait) != 0)
> +		goto retry;
> +	if (TYPE(a.buf) != T_SYMBOL)
> +		kgio_autopush_write(io);
> +	return a.buf;

(I'm not fully awake): You do retry when truncating to iov_max, right?

> +}
> +#endif
> +
> +/*
> + * call-seq:
> + *
> + *	io.kgio_writev(array)	-> nil
> + *
> + * Returns nil when the write completes.
> + *
> + * This may block and call any method defined to +kgio_wait_writable+
> + * for the class.
> + *
> + * It fallbacks to kgio_write when writev(2) syscall is missing
> + */
> +static VALUE kgio_writev(VALUE io, VALUE ary)
> +{
> +	VALUE array = rb_check_array_type(ary);
> +#ifdef HAVE_WRITEV
> +	return my_writev(io, array, 1);
> +#else
> +	VALUE str = rb_ary_join(array, Qnil);
> +	return my_write(io, str, 1);
> +#endif
> +}

I don't like #ifdef inside function definitions, so I'd rather have
something like this:

	#ifdef HAVE_WRITEV
	static VALUE kgio_writev(VALUE io, VALUE ary)
	{
		...
	}
	#else
	#  include "compat_kgio_writev.h"
	#endif

> +static VALUE kgio_trywritev(VALUE io, VALUE ary)
> +{
> +	VALUE array = rb_check_array_type(ary);
> +#ifdef HAVE_WRITEV
> +	return my_writev(io, array, 0);
> +#else
> +	VALUE str = rb_ary_join(array, Qnil);
> +	VALUE result = my_write(io, str, 0);
> +	if (TYPE(result) == T_STRING) {
> +		return rb_ary_new4(1, &result);
> +	}
> +	return result;
> +#endif

Ditto.