linux-kernel - Re: Higher than expected disk write(2) latency

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Message-ID: <4867733A.3060203@fastmq.com>
Date:	Sun, 29 Jun 2008 13:34:18 +0200
From:	Martin Sustrik <sustrik@...tmq.com>
To:	David Newall <davidn@...idnewall.com>
CC:	Martin Lucina <mato@...elna.sk>, linux-kernel@...r.kernel.org
Subject: Re: Higher than expected disk write(2) latency

David Newall wrote:
> Martin Lucina wrote:
>> Measurement of the write(2) time is performed using the TSC, so any
>> latency there is negligible.
> 
> Measuring each write is lavish and bound to impact the results.  To
> measure sustained throughput, get the time only at start and end, and
> divide the difference by the number of writes.

Hi,

We're using RDTSC to measure time, which takes ~30 nanoseconds to 
complete. I would say this is negligible given that latency of a write 
is 5 orders of magnitude higher.

Martin

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