How to reduce the latency of FFmpeg h264_qsv encoder? - ffmpeg

Question

I use FFmpeg(3.4) h264_qsv encoder in my camera-live software, I found that the encoded data recieved after sending 40 frames! There is about 1.5 seconds latency, it is not sutiable for realtime situation.How to config the encoder? Thanks a lot.
my code:
AVCodecContext* OpenH264Codec(int width, int height, int bitrate, int framerate)
{
AVCodecContext* ctx = 0;
AVCodec* c = 0;
c = avcodec_find_encoder_by_name("h264_qsv");
if (c == NULL) return NULL;
ctx = avcodec_alloc_context3(c);
if (ctx == NULL) return NULL;
ctx->width = width;
ctx->height = height;
ctx->pix_fmt = c->pix_fmts[0];
ctx->bit_rate = bitrate;
ctx->bit_rate_tolerance = ctx->bit_rate / 2;
ctx->rc_min_rate = 32000;
ctx->rc_max_rate = ctx->bit_rate*1.5;
ctx->time_base.den = framerate;
ctx->time_base.num = 1;
ctx->framerate.den = 1;
ctx->framerate.num = framerate;
ctx->gop_size = framerate*5;
ctx->max_b_frames = 0;
av_opt_set(ctx->priv_data, "preset", "veryfast", 0);
av_opt_set(ctx->priv_data, "avbr_accuracy", "1", 0);
av_opt_set(ctx->priv_data, "async_depth", "1", 0);
av_opt_set(ctx->priv_data, "profile", "main", 0);
ctx->flags |= AV_CODEC_FLAG_QSCALE;
if (avcodec_open2(ctx, c, 0) < 0)
{
avcodec_free_context(&ctx);
return NULL;
}
return ctx;
}

Answer 1

Thanks for Mulvya's tip. There is only 5 frames latency now.
AVCodecContext* OpenH264Codec(int width, int height, int bitrate, int framerate)
{
AVCodecContext* ctx = 0;
AVCodec* c = 0;
c = avcodec_find_encoder_by_name("h264_qsv");
if (c == NULL) return NULL;
ctx = avcodec_alloc_context3(c);
if (ctx == NULL) return NULL;
ctx->width = width;
ctx->height = height;
ctx->pix_fmt = c->pix_fmts[0];
ctx->bit_rate = bitrate;
ctx->bit_rate_tolerance = ctx->bit_rate / 2;
ctx->time_base.den = framerate;
ctx->time_base.num = 1;
ctx->framerate.den = 1;
ctx->framerate.num = framerate;
ctx->gop_size = framerate*5;
ctx->max_b_frames = 0;
av_opt_set(ctx->priv_data, "preset", "medium", 0);
av_opt_set(ctx->priv_data, "look_ahead", "0", 0);
//av_opt_set(ctx->priv_data, "look_ahead_depth", "8", 0);
if (avcodec_open2(ctx, c, 0) < 0)
{
avcodec_free_context(&ctx);
return NULL;
}
return ctx;
}