Re: [RFC][PATCH 0/3] VM throttling: avoid blocking occasional writers

[Yuji Kakutani]

Hi,
Thank you for your comments.

Leroy van Logchem wrote:
>The default dirty_ratio on most 2.6 kernels tend to be too large imo.
>If you are going to do sustained writes multiple times the size of
>the memory you have at least two problems.
>
>1) The precious dentry and inodecache will be dropped leaving you with
>a *very* unresponsive system
>2) The amount of dirty_pages which need to be flushed to disk is huge,
>taking all VM while the i/o channel takes uninterruptable time to　flush it.

I agree that the default dirty_ratio is too large.
What I have observed is that write(2) to a disk only with light load is
blocked in balance_dirty_pages() due to heavy write to *another* disk,
and doesn't seem to be the same issue as you say. But decreasing
dirty_ratio shortens the blocking time also in our experiments.

>What we really need is a 'cfq' for all processes -especially misbehaving
>ones like dd if=/dev/zero of=/location/large bs=1M count=10000-.
>If you want to DoS the 2.6 kernel, start a ever running dd write and
>you know what I mean. Huge latencies due the fact that all name_to_inode
>caches are lost and have to be fetched from disk again only to be quickly
>flushed again and again. I already explained this disaster scenario with
>Linus, Andrew and Jens; hoping for a auto-tuning solution which takes
>diskspeed per partition into account.

I appreciate for your information.
Would you know about any progress in this issue?


Regards,
-- 
Tomoki Sekiyama & Yuji Kakutani
Hitachi, Ltd., Systems Development Laboratory

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