Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752175Ab0LZM4f (ORCPT ); Sun, 26 Dec 2010 07:56:35 -0500 Received: from mail-wy0-f174.google.com ([74.125.82.174]:59283 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751813Ab0LZM4e convert rfc822-to-8bit (ORCPT ); Sun, 26 Dec 2010 07:56:34 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=fBvF8tksZpflO+JgR9FmesTM9Ewa2kyKUKKpzYl4IlxtyJpltMLzToh5Au2jWCOzHt /8ptxX1EzlSE015dxPArLcRFZJWTg6t0udULSo/QPRXTxRlCY6kdn0S1116NIytSrhmR gaRnrt/9BEM000DYZrO3YqVqQVgOw9Di1ZQrA= MIME-Version: 1.0 In-Reply-To: References: Date: Sun, 26 Dec 2010 18:56:32 +0600 Message-ID: Subject: Re: [PATCH v0] add nano semaphore in kernel From: Rakib Mullick To: Hillf Danton Cc: linux-kernel@vger.kernel.org, mingo@redhat.com Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2761 Lines: 63 On Sun, Dec 26, 2010 at 6:05 PM, Hillf Danton wrote: > On Sun, Dec 26, 2010 at 5:08 PM, Rakib Mullick wrote: >> On Sun, Dec 26, 2010 at 1:04 PM, Hillf Danton wrote: >>> On Sun, Dec 26, 2010 at 2:46 PM, Rakib Mullick wrote: >>>> On Sun, Dec 26, 2010 at 11:13 AM, Hillf Danton wrote: >>>> Above description tells how its done, what it is but its not clear why >>>> we should use it. Can I know why should we use it or its benefits? >>>> >>> >>> The outstanding benefit looks that nano semaphore could be used in >>> cases where callers want to wait not only more than one jiffy, but far >>> less than one jiffy also. >>> >> But, how do we know that resources are going to be available within >> one jiffy or far less than one jiffy? We we be deterministic? >> > This is a really hard question. > > Though I could not answer as fine as Ingo Molnar could, the > deterministic has to be faced by most contentions for resources in > kernel, simply because deadlock could occur in spin lock for instance. > Yes, if its not handled carefully. > On the other hand, this question explores a byproduct advantage, not > seriously considered before, ?of nano semaphore, that the caller could > learn that there is something out of track if she waited over 100 > microseconds in 5 consequent steps, 20 microseconds a step, if she > think the resource should be available in 60 microseconds. > Why should caller think such a way, that resource will be available in 60 microseconds? > And in nano semaphore method is available for callers to select > waiting over one second, one millisecond, one microsecond, one > nanosecond, depending on what the underlying system could offer. After > waiting, the caller is free to determine what to do next if the > resource is not available. > How do we know that, what the underlying system will offer? Its an NP-type problem. We cannot determine what will happen on a systems context. When a caller need resource, without that resource is it possible to accomplish its job? If its not, then the ultimate way to deal with it, is simply waiting until it gets the resource. > Here is another sample, if taxi will not come in two minutes, I could > either give up shopping downtown, or wait another three minutes, > depending on what will happen two minutes later. But what, if your mother told you not to come home until you done shopping? :) thanks, rakib > > thanks > Hillf > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/