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From: Pavel Begunkov <[email protected]>
To: JackieLiu <[email protected]>
Cc: [email protected], [email protected]
Subject: Re: [PATCH RESEND] io_uring: a small optimization for REQ_F_DRAIN_LINK
Date: Fri, 22 Nov 2019 13:54:55 +0300	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 11/22/2019 1:26 PM, JackieLiu wrote:
>> Not sure about that. It's 1 CMP + 1 SETcc/STORE, which works pretty fast
>> as @drain_next is hot (especially after read) and there is no write-read
>> dependency close. For yours, there is likely always 3 CMPs in the way.
>>
>> Did you benchmarked it somehow or compared assembly?
> 
> It is only theoretically possible. In most cases, our drain_link 
> and drain_next are both false, so only two CMPs are needed, and modern CPUs
> have branch predictions. Perhaps these judgments can be optimized.
> 
My bad, right, 2 CMPs in the common way.

> Your code is very nice, when I reading and understanding your code,
> I want to try if there is any other way to optimize it. 
> 
> Sometimes you don't need to reset drain_next, such as drain_link == true && 
> drain_next == true, you don't need to set below one more time.

We may think to change like below, but I'd rather rely on a compiler to
optimise it for us (i.e. knowing the target architecture). Everything
else is a really rare/slow path in my opinion, so shouldn't be of concern.

-	req->ctx->drain_next = (req->flags & REQ_F_DRAIN_LINK);
+	if (req->flags & REQ_F_DRAIN_LINK)
+		req->ctx->drain_next = true;

If the goal is to micro-optimise things, it's better to think how to
toss the whole scheme to reduce number of CMPs and memory read/writes in
the hot path, including setting REQ_F_DRAIN_LINK in submit_sqe().

Though, there are still heavier things happening around.

-- 
Pavel Begunkov

      reply	other threads:[~2019-11-22 11:13 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-22  6:01 [PATCH liburing RESEND] Update link_drain with new kernel method Jackie Liu
2019-11-22  6:01 ` [PATCH RESEND] io_uring: a small optimization for REQ_F_DRAIN_LINK Jackie Liu
2019-11-22 10:05   ` Pavel Begunkov
2019-11-22 10:26     ` JackieLiu
2019-11-22 10:54       ` Pavel Begunkov [this message]

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